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THE  EVOLUTION  OF  MAN 

Vol.  I.  -HUMAN  EMBRYOLOGY  OR  ONTOGENY 


THE  EMRRYONIC  DEVELOPMENT  OF  THE  FACE 


The  Evolution  ai    \d<m  I  Ed 


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THE  EVOLUTION 

OF  MAN 

A    POPULAR   SCIENTIFIC   STUDY 

LIBRARY 
k.  w  V.,f  OLLEGBL 

ERNST    HAECKEL 


Vol.  I. 
HUMAN  EMBRYOLOGY  OR  ONTOGENY 


Translated  from  ///,   Fifth  {enlarged)  Edition  by  JOSEPH  McCABE 


Nkw  York:    G.  P.  PUTNAM'S  SONS 

London  :  WATTS  &  CO. 

•9°5 


CONTENTS 


Lisi  of  Plates 

Lisi  of   1-  u. i  res  in  Text 

Lisi   01    Tables 

I'm  i  u  e  ro  Foi  r  in  Edition 

I'ki  i  mi    u>  Fifth  Edition 


Vol.  I.— EMBRYOLOGY  (ONTOGENY) 

I.  I'm   Fundamental  Law  of  Organic  Evolution        -           -  i 

II.  The  Older  Embryology            .....  21 

III.  Modern  Embryology      -.-...  37 

IV.  The  Older  Phylogeny  ------  w 

V.  The  Modern  Science  of  Evoli  tion    -  75 

VI.  Till    Om  M    VND  THE  AMCEBA           -----  q(, 

VII.  Conception           -           -           -           -           -           -           -  U4 

VIII.  Tin  Gastrjea  Theory     ------  [^g 

IX.  The  Gastrulation  of  rHE  Vertebrate          -           -           -  174 

X.  The  Ccelom  Theory        ------  >i& 

XI.  The  Vertebrate  Character  01  Man  ...           -  24(1 

XII.  Embryonic  Shield  and  Germinative  Area     -           -           -  273 

XIII.  Dorsal  Body  wd  Ventral  Body         -  .2^4 

XIV.  The  Articulation  of  the  Body           ....  330 
XV.  FtETAL  Membranes  and  Circulation    ...           -  361 


*^ 


'oe 


LIST  OF   PLATES  IX  Vol.   I. 

Plate    I.     Embryology    >>i     iiii     Him  an    FaCE.     (Frontispiece   to   firsl 
volume)  -  -  Explanation  see  Chapter  XXV.,  Vol.  II. 

rial.-    II.     Gastrulation    01     Holoblastic     Animals    (with    total    seg- 
mentation)      ---...         Explanation  p.      170 
Plate  III.     Gastrixation  >>i    Mesoblasth    Animals  (with  partial  seg- 
mentation)      ----..        Explanation  p.     17.1 
Plate    IV.     Sandal-Shaped    Embryos    01     Three    Sauropsids   (lizard, 

tortoise,  hen)  at  three  different  stages         -  -         Explanation  p.      ;,, ; 

Plate  V.     Sandal-Shaped  Embryos  of  Threi   Mammals  (pig,  hare,  man) 

at  three  different  stages         -  Explanation  p.      ;.,.; 

Plate  VI.     Transversi    mmiunm'!   V'er ;\ii    Embryos.     Diagram- 
matic, germ-layers  coloured             -  -  -Explanation  pp.  326   j2g 
Plate   VII.     Longitudinal  Sections   .>i    V'ertebrati    Embryos.     Dia- 
grammatic, germ-layers  coloured     -           -           -Explanation  pp.  326-329 
Plate  \  III.     Embryos  Of  Three  Reptiles  (lizard,  serpent,  crocodile)  at 

three  different  stages  -  Explanation  p.     359 

Plate  IX.     Embryos  01    Threi    Sacropsids   (tortoise,  hen,  ostrich)  at 

three  different  stages  -  Explanation  p.     359 

Plate  X.     Embryos  01   Three  Sai  ropsids  (stem-reptile,   river-tortoise, 

kiwi)  at  three  different  stages  -  -  -         Explanation  p.     359 

Plate  XI.     Embryos  01    Three    Mammals  (hedge-hog,  dolphin,  gibbon) 

ai  three  different  stages      -  Explanation  p.     359 

Plate  XII.     Embryos  01    Four   Mammals  (opossum,   pig,  goat,   ox)  al 

three  different  stage  -  Explanation  p.     359 

Plate  XIII.     Embryos  01   Four  Mammals  (dog,  bat,  hare,  man)  at  three 

different  stages  .....         Explanation  p.      J59 

Plate   XIV.     Human    Embryos   From    iiii     Fourth    ro    1111:    Eighth 

"  '  '  K  ----.-_     Explanation  pp.    5(14  ^ 
Plate  XV.     Human  Embryo  in  iiii   Fojtal  Membranes  (from  the  second 

to  the  twelfth  week)  ....        Explanation  p.     411 

Plate   XVI.     Human     Embryo,     Five    Months    Old,   in    iiii,   I-'h-iai. 

Membranes    --....        Explanation  p.     411 


LIST  OF  FIGURES  IN  THE  TEXT 


The  human  ovum 
Stem-cell    of    one    of   the 

echinoderms   - 
Throe  epithelial  cells  - 
Five  spiny  or  grooved  cells 
Ton  liver  cells      - 

Nine  star-shaped  bone-cells 
Eleven  star-shaped  tells  - 
Unfertilised    ovum    of    an 

echinoderm     - 
Large  branching  nerve-cell 
Blood-cells  - 

Indirect      or    mitotic    cell- 
division    - 
Mobile    cells    from    the    in- 
flamed eye  of  a  frog 
Ova  of  various  animals 
The  human  ovum 
Fertilised    ovum    from    the 

oviduct  of  a  hen 
A  creeping  amoeba     - 
Division    of    a     unicellular 

amoeba    - 
Ovum  of  a  sponge 
Blood-cells,  or  phagocytes 
Spermia  or  spermatozoa     - 
Spermatozoa      of     various 

animals    - 
A  single  human  spermato- 
zoon        - 
Fertilisation  of  the  ovum    - 
Unfertilised    ovum     of    an 

echinoderm      - 
Impregnated      echinoderm 

ovum        - 
Impregnation  of  the  ovum 
of  a  star-fish   - 
28.      Impregnation     of     the 
ovum  of  the  sea-urchin  - 
Stem-cell  of  a  sea-urchin    - 
Stem-cell  of  a  hare      - 
Gastrula  of  a  coral     - 
Gastrula  of  a  gastraead 
Gastrula  of  a  worm     - 
Gastrula  of  an  echinoderm 


PICt 

97 

35- 

36. 

99 

.;:■ 

100 

38. 

1O0 

39- 

58. 


129 

'3° 

59- 

60. 

l33 

61. 

'33 

62. 

'35 

63- 

136 

'37 

64. 

'43 

'S° 

65- 

'53 

66. 

'53 

67. 

'53 

68. 

Gastrula  of  an  arthropod  - 
Gastrula  o(  a  mollusc 
Gastrula  of  a  vertebrate  - 
Gastrula  of  a  lower  sponge 
Cells  from  the  primary  ger- 
minal layers  -  -  - 
Gastrulation  of  the  amphi- 

oxus  .... 

Gastrula  of  the  amphioxus 

Cleavage  of  the  frog's  ovum 

46.      Sections  of  the  fertilised 

ovum  of  the  toad    - 

Embryonic   vesicle   of   the 

water-salamander  - 
Embryonic  vesicle  of  triton 
Sagittal  section  of  a  hooded- 

embryo  of  triton     - 
Section  of  the   gastrula  of 

triton  - 
Segmentation  in  the  lamprey 
Gastrulation  of  the  lamprey 
Gastrulation  of  ceratodus  - 
Ovum  of  a  pelagic  bony  fish 
Segmentation  of  a  bonv  fish 
Discoid  gastrula  of  a  bony 

fish 

Section  of  the  blastula  of  a 

shark       - 
Section  of  the  blastula  of  a 

shark        - 
-Mature  ovum  of  a  hen 
Diagram    of    discoid    seg- 
mentation        -         -         - 
Section  of  the  blastula  of  a 
hen  ---.. 
Germinal   disk  of  the  hen's 
ovum       -        -        _        _ 
Section     of     the     germinal 

disk  of  a  siskin 
Section  of  the  discoid  gas- 
trula of   the  nightingale 
Germinal  disk  of  the  lizard 
Ovum  of  the  opossum 
Blastula  of  the  opossum 
Blastula  of  the  opossum      - 


'53 
'53 
'53 
'56 

'58 

'59 
160 
176 


181 

1S2 


'83 

184 
184 
186 
1 88 
189 

190 
'93 

'94 
196 

'97 

'99 

'99 

199 

200 
202 

204 
2°5 


LIST  OF  FIGURES  IN  THE  TEXT 


76. 


Gastrula  of  the  opossum  - 
Section  of  the  gastrula  of 
the  opossum  -  -  - 
Stem-cell  of  th"  mammal 
ovum  -  -  -  - 
Incipient    cleavage   of  the 

mammal  ovum 
First  segmentation-cells  ol 

the  mammal  ovum  - 
Mammal    ovum  with  eight 

segmentation-cells  - 
Gastrula   of    the    placental 
mammal  -         -         -         - 
Gastrula  of  the  hare  - 
78.       Diagram    of    the    four 
secondary  germ-layers  - 
80.     Ccelomula  of  sagitta     - 
Section  of  a  young  sagitta 
83.     Section    of  amphioxus- 
larvs       -        -        -        - 
85.     Section    of   amphioxus 
embryo    -         -         -         - 
87.     Chordula  of  the  amphi- 
oxus        -        -        -        - 
Sg.      Chordula    of     the    am- 
phibia      - 
91.      Section     of    coelomula- 
embryos  of  vertebrates  - 
93.      Section     of     ccelomula- 
triton        -         -         -         - 
Section     of     three     triton- 
embryos  -         -         -         - 
Section    of    the    chordula- 

embrvo  of  a  bird     - 

Section  of  the   vertebrella- 

embryo  of  a  bird     - 

9S.     Section  of  the  primitive 

streak  of  the  chick  - 

Section    of    the    primitive 

groove  of  a  hare 

Section    of    the    primitive 

mouth  of  a  human  embryo 

-105.       The     ideal     primitive 

vertebrate        -         -         - 

Instances       of      redundant 

mammary  glands    - 
A  Greek  gynecomast 
,      Severance    of    the    discoid 

mammal  embryo      - 

,    no.    The  visceral  embryonic 

vesicle  of  a  hare 

Four  entodermic  cells 

Two  entodermic  cells 

-[17.      Ovum  of  a  hare    - 


>o6     118. 


208 

208 

123. 

208 

124. 

J  09 

>-5- 

210 

126. 

219 

127. 

227    129. 

130. 
22S 

'3-- 
230 

^3°     '34- 
'35- 


,38. 

■39' 

1 40. 


240 

^53 

'43- 

266 

268 

148- 

277 

152. 

2S1 
282 
282 
28s 

'53- 

'54- 
'55- 

Round  germinative  area  of 

the  hare  -  -  -  -  286 
Oval  area    -  286 

Oval  germinal  disk  of  the 

hare  -  288 

Pear-shaped  germinal  shield 

of  the  hare       -  288 

Section   of  the  gastrula   of 

four  vertebrates  -  -  291 
Embryonic  vesicle  of  a  hare  295 
Oval    embryonic    shield    of 

the  hare  -  295 

Dorsal    shield    of    a     hare- 
embryo    -         -         -         -     296 
Embryonic  shield  of  a  hare     297 
Section  of  the  ccelomula  of 

amphioxus        -  297 

Section  of  the  chordula   of 

a  frog  -  -  -  -  298 
Section  of  a  frog-embryo  -  298 
131.      Dorsal    shield    of    the 

chick        -  -         -         -     ^99 

Section  of  the  hinder  end  of 

a  chick  -  -  -  -  300 
Germinal  area  of  the  hare  -  300 
Embryo  of  the  opossum  -  301 
Sandal-shaped    embryonic 

shield  of  a  hare  -  -  301 
Human      embryo      at      the 

sandal-stage  -  -  -  3°' 
Sandal-shaped     embryonic 

shield  of  a  hare  -  -  302 
Sandal-shaped    embryonic 

shield  of  an  opossum  -  304 
Section    of   the   embryonic 

shield  of  a  chick  -  -  306 
Section    of  the   embryonic 

disk  of  a  chick  -  -  306 
Section    of  the   embryonic 

shield  of  a  chick      -         -     307 
Sections  of  the  embryonic 
disk  of  the  higher  verte- 
brates      -         -         -         -     309 
147.    Sections  of  the  maturing 
mammal  embryo  and   its 
envelopes         -         -         -     310 
151.      Sections    of  chick-em- 
bryos       -         -         -         -     3 '4 
Section  of  the  embryo  of  a 

chick  -  -  -  -317 
Section  of  the  fore-half  of  a 

chick-embryo-  -  -  318 
Section  of  a  human  embryo  320 
Section  of  a  human  embryo     321 


LIST  OF  FIGURES  IX  THE  TEXT 


F1GUKE 

156.  Section  of  a  shark  embryo 

157.  Section  of  a  dink  embryo  - 
158  [60.     Embryonic  disk  of  the 

chick       - 

161.  Embryo  of  the  amphioxus  - 

162,  163.     Embryo  of  the  amphi- 

oxus       -        -        -        . 
164-160.     Embryo  of  the  amphi- 
oxus       - 

167.  Section   of   an    amphioxus 

embryo    - 

168,  169.     Section  of  shark  em- 

bryos      -        -        -        - 

170.  Frontal  section   of  a  triton 

embryo    -         -         -         - 

171.  Section  of  a  chick  embryo - 

172.  Section  of  a  chick  embryo - 

173.  The  third  cervical  vertebra 

174.  Tin'  sixth  dorsal  vertebra  - 

175.  The  .second  lumbar  vertebra 

176.  Section   of   the   trunk   of  a 

primitive  vertebrate 

177.  Section     of    the     primitive 

vertebrate  - 
17S.  Head  of  a  shark  embryo  - 
179,  tSo.  Head  of  a  chick  em- 
bryo .... 
181.  Head  of  a  dog  embryo 
1S2.  Human  embryo  ofthefourth 
week         _  _  -  - 

183.  Section  of  shoulder  of  chick 

embryo    ...         - 

184.  Section  of  pelvic  region  of 

chick  embryo  ... 

185.  Development  of  the  lizard's 

legs         - 

186.  Human  embryo,  five  weeks 

old  - 
[87    189.      Embryos  of  the  bat 

190.  Sandal-shaped    human   em- 

bryo .         .         _         . 

191.  Human        embryos        from 

second  to   fifteenth  week 

192.  Human    embryo    of  fourth 

« eek       -        -        -        - 

193.  Human     embryo      of     fifth 

week        .... 

194.  Section  of  tail  of  a  human 

embryo    - 

195.  Tail    of    a     six-months'-old 

boy  -        -        -        -        - 

196.  Human  embryo,  four  weeks 

old  -        -'      - 

197.  Human  embryo,  five  weeks 


3a> 

IE 

old 

37o 

J-- 

1 98. 

Head  of  Miss  Julia  Pastrana 

372 

199. 

Human  ovum  (twelve  days) 

374 

334 

200. 

Human  ovum  (ten  days) 

374 

336 

201. 

Human  foetus  (ten  days)     - 

374 

202. 

Human    ovum    (twenty    to 

337 

twenty-two  days)     - 

374 

203. 

Human    foetus   (twenty   to 

338 

twenty-two  days)    - 

374 

204. 

Human  embryo  of  sixteen 

339 

to  eighteen  days     - 

375 

205. 

Human    embryo    of   fourth 

34° 

week         ...         - 

376 

206. 

Human    embryo    of   fourth 

34' 

week        - 

376 

34' 

207. 

Human     embryo     with     its 

342 

membranes      - 

377 

344 

208. 

Section  of  embryo  of  a  chick 

377 

344 

209. 

Embryonic    organs    of   the 

344 

mammal  -         -         -         - 

37S 

210. 

Embryo  of  a  dog 

379 

347 

211. 

Embryo  of  a  dog 

380 

212. 

Section     of     the     pregnant 

348 

human  womb  - 

3s ' 

35° 

^'3- 

215.    Embryos  ofthekalawet- 

gibbon     -         -         -         - 

382 

35° 

216. 

Male     embryo    of    the    sia- 

35° 

mang-gibbon  -         -         - 

3S3 

217. 

Section  of  pregnant  human 

35^ 

womb       -         -         -         - 

384 

218. 

Human  foetus,  twelve  weeks 

353 

old  ----- 

385 

219. 

Mature  human  foetus  - 

386 

354 

220. 

Section  of  the  lower  half  of 
the  trunk  of  a  woman  in 

355 

advanced  pregnancy 

3S7 

221- 

225.    Sections  of  the  maturing 

356 

mammal  embryo 

389 

357 

226. 

Section  of  the  embryo  of  a 

chick         - 

39' 

3<>4 

227. 

Section  of  the  embryo  of  a 

chick         -         -      '  - 

39' 

365 

22S. 

Section  of  the  embryo  of  a 

chick        - 

392 

366 

229. 

Human  embryo  (fourteen  to 

eighteen  days) 

393 

366 

230. 

Section    of   the    bead    of  a 

mammal  embryo 

394 

368 

23'- 

Vitelline  vessels  in  the  ger- 

minative  area  of  a  chick 

369 

embryo    -         -         -         - 

395 

232- 

Boat-shaped  embryo  of  the 

37° 

dog           -         -         -         - 

396 

-Wv 

Embryonic  shield  of  a  hare 

397 

LIST  OF  FIGURES  IN  THE  TEXT 


234- 
235- 
236. 

237- 


Embryonic  shield  of  a  hare 
Lar  or  white-handed  gibbon 
Young  orang  -  -  - 
Wild  orang  -        -        - 

Head  of  an  old  male  orang     403 
The  bald-headed  chimpanzee  404  I 


M ,  B 

FILL 

RE 

1  \1.1-- 

398 

J  4O 

Female  chimpanzee     - 

-     405 

41  !<) 

241 

Female  mafuka  - 

-    406 

40I 

242 

Female  gorilla     - 

-     4°7 

402 

243 

Male  giant-gorilla 

-     408 

403 

244 

Giant-gorilla 

-     40b 

LIST  Ob'  GENETIC  TABLES 


i.  Composition  of  the  organic  cell  -        - ,,- 

2.  Differences  in  segmentation  and  gastrulation ,-, 

3.  Four  embryonal  stages  in  animals      -        -        -        -        .        _  ,-, 

4.  Chief  variations  in  segmentation ,«, 

5.  Phytogeny  of  vertebrate  gastrulation  -        ------  ,,, 

6.  Four  types  of  vertebrate  gastrulation          ------  21s 

7.  Names  of  the  germinal  layers      --- ->,•> 

S.  Origin  and  function  of  the  fundamental  organs  of  the  chordula         -  243 

9.  Fundamental  organs  and  body-cavities  of  the  chordula      -        -        -  244 

0.  Four  chief  groups  of  the  metazoa        ----...  24- 

1.  Chief  organs  of  the  provertebrates     -        -        -         -        -        .  1-, 

2.  Composition  of  the  amniote  embryo 2q? 

3.  Composition  of  the  vertebrate  body    ------  n2. 

4.  Organisation  and  articulation  of  the  vertebrates  and  articulates      -  360 

5.  Embryonic  plates  of  the  vertebrates -  410 


PREFACE  TO  THE   FOURTH   EDITION' 


WHEN  the  first  edition  of  this  work  appeared  in  1874,  and 
the  third  edition  followed  three  years  afterwards,  the  circum- 
stances of  biologv  were  very  different  from  what  they  are 
to-day.  It  is  true  that  the  struggle  for  the  recognition  of  the 
great  truths  of  science,  which  Darwin  had  initiated  by  the 
publication  of  his  epoch-making  work  in  1859,  had  already 
been  decided  in  his  favour  on  the  main  issue.  But  the  most 
important  consequence  of  the  new  evolutionary  doctrine  (now 
firmly  established  for  the  first  time  through  his  theory  of 
selection) — that  is  to  say,  its  application  to  man— still  met 
with  the  most  spirited  and  widespread  opposition. 

I  had  in  my  Generelle  Morphologies  published  in  1866, 
made  the  first  attempt  to  trace  the  series  of  man's  ancestors, 
and  to  indicate  the  several  stages  of  animal  organisation 
which  led  up  to  his  appearance  ;  and  I  had  continued  this 
task  in  my  History  of  Creation,  published  in  1868.  The 
profound  importance  that  the  facts  of  human  embryology 
have  in  the  attempt  to  construct  our  ancestral  tree  became 
more  and  more  evident  to  me.  A  prolonged  study  of  human 
embryology,  and  the  giving  of  university  lectures  on  this 
first  base  of  physical  anthropology,  emboldened  me  to  attack 
the  difficult  task  of  applying  it  to  the  history  of  our  species. 

The  complete  application  to  man  of  the  first  law  of  biogeny 
seemed  to  me  the  more  useful  and  desirable  as  the  great 
majority  of  embrvologists  at  that  time  knew  nothing  about  it. 
The  only  work  that  dealt  comprehensively  with  human  embryo- 
logy after  1859 — namelv,  Albert  Kolliker's  widely-circulated 
Manual — took  an  entirely  opposite  view  ;  even  in  the  latest 
edition  (1884)  the  distinguished  author  adheres  to  the  opinion 

1  Nol  translated  into  English. 


PREFACE  TO  THE  FOURTH  EDIT/OX 


that  "  the  laws  governing  the  evolution  of  living  things  are 
still  wholly  unknown  ;  it  is  believed  that  the  development 
took  place  by  abrupt  stages  rather  than  by  a  continuous 
growth,  as  the  Darwinians  imagine." 

In  opposition  to  this  dualistic  idea  that  was  then  prevalent 
on  all  sides,  I  attempted  in  1874  to  obtain  a  hearing  for  my 
monistic  conception  of  the  embryological  phenomena.  I 
started  from  the  following  general  principles  : — 

1.  There  is  a  direct  causal  connection  between  the 
observed  facts  of  human  embryology  and  the  theoretical 
ancestry  of  our  race,  which,  for  obvious  reasons,  for  the  most 
part  lies  outside  our  sphere  of  observation. 

2.  This  mechanical  causal  nexus  finds  its  simplest 
expression  in  the  fundamental  law  of  biogeny:  "Ontogeny  is 
a  brief  and  imperfect  recapitulation  of  phylogeny."  ' 

3.  The  phylogenetic  process,  or  the  gradual  development 
of  man's  higher  vertebral  ancestors  through  a  long  series  of 
lower  animal  forms,  is  a  very  complex  historical  fact,  due  to 
a  manifold  play  of  heredity  and  adaptation. 

4.  Each  one  of  the  processes  involved  depends  on  the 
physiological  functions  of  the  organism,  and  can  be  traced  to 
the  action  of  either  reproduction  (heredity)  or  nutrition 
(adaptation). 

5.  The  fact  of  human  embryology  can  only  be  explained 
as  the  inheriting  of  phylogenetic  (ancestral)  forms,  in  which 
the  palingenetic  phenomena  are  to  be  carefully  distinguished 
from  the  cenogenetic.2 

6.  Only  the  palingenetic  phenomena  (that  is  to  say,  such 
reminiscences  of  earlier  stages  as  the  temporary  formation  of 
the  spinal  cord,  the  primitive  kidneys,  or  the  gill-clefts)  are  of 
direc'  interest  in  the  tracing  of  our  animal  ancestors,  because 
they  are  due  to  the  inheritance  of  adaptive  structures  in 
earlier  animals. 

1  Biogeny  is  the  general  science  ot  the  development  of  lite  ;  ontogeny  is  the 
genesis  of  the  individual  (or  the  science  dealing  with  this — embryology);  and 
phytogeny  the  genesis  of  the  species.  Further  explanation  will  be  given 
presently. — Trans. 

-  Palingenesis  =  "  repeated  "  or  inherited  evolutionary  phenomena  :  eeno- 
genesis  =  ''foreign,"  or  more  recently  acquired  phenomena. — TRANS. 


PREFACE  TO  THE  FOURTH  EDITIOX 


-.  On  the  other  hand,  the  cenogenetic  phenomena  (such 
as,  for  instance,  the  embryonic  formation  of  the  foetal 
membranes,  the  allantois,  the  dual  structure  of  the  heart, 
etc.)  have  only  a  subordinate  and  indirect  interest  for 
phytogeny,  as  they  have  arisen  later  by  the  adaptation  of  the 
foetus  to  its  embryonic  conditions. 

S.  The  many  gaps  in  phytogeny,  which  are  due  to  the 
lack  of  empirical  material  in  embryology,  may  be  remedied  for 
the  most  part  from  paleontology  and  comparative  anatomy. 

The  application  of  these  general  principles  of  biogeny  to 
the  particular  case  of  the  evolution  of  man,  as  I  first  attempted 
it  in  my  Anthropogenie,  was  bound,  oi  course — being  the 
earliest  independent  advance  into  a  fresh  field  of  investiga- 
tion— to  be  imperfect.  At  the  most  it  could  only  hope  to 
attract  attention  to  this  new  inquiry,  and  to  induce  other 
Students  to  test  the  results  in  their  special  provinces.  When 
we  compare  the  condition  of  our  science  at  that  time  with  its 
situation  to-day,  I  think  we  must  admit  that  my  Anthropo- 
genie  fully  achieved  its  aim  in  this  respect.  Most  men  of 
science  who  have  since  worked  in  the  field  of  comparative 
evolution  are  convinced  to-dav  that  the  two  chief  sections  of 
it  which  I  was  the  first  to  distinguish — Ontogeny  and 
Phytogeny — have  a  causal  connection  of  the  closest 
character,  and  that  the  one  cannot  be  understood  apart  from 
the  other.  The  great  majority  of  the  useful  results  which 
their  sedulous  and  searching  inquiries  have  yielded  can  only 
be  thoroughly  appreciated  when  we  recognise  that  the  facts 
of  ontogeny  have  found  an  explanation  in  phytogeny. 
Twenty-five  vears  ago,  when  my  Generelle  Morphologic 
appeared,  human  embryology  was  generally  looked  upon 
as  a  sort  of  fairyland,  in  which  a  number  of  most  extra- 
ordinary and  enigmatic  processes  were  linked  together 
without  any  visible  ground  in  the  shape  of  causal  connection. 
To-day,  on  the  contrary,  we  see  in  this  chain  of  wonderful 
processes  an  historical  document  of  the  first  importance,  a 
chapter  of  the  story  of  creation,  which  gives  us  most  valuable 
information  as  to  the  chief  features  of  the  bodily  structure  and 
mode  of  life  of  our  animal  ancestors. 


PREFACE  TO  THE  FOURTH  EDITION 


The  brilliant  progress  that  comparative  embryology  has 
made  during  the  last  few  decades  is  often  attributed  to 
extrinsic  considerations — to  the  great  number  of  fresh 
workers  in  this  field  of  research,  and  to  the  improvement  in 
the  technical  methods  of  investigation  and  the  instruments 
used  in  the  study.  Certainly  we  must  not  fail  to  appreciate 
these  advantages,  especially  the  improvement  of  the  micro- 
scope and  microtome  ;  but  the  chief  cause  of  progress  has 
been  the  application  of  phylogenetic  methods.  It  is  to  this 
we  owe  that  immense  enlargement  of  our  intellectual  horizon 
which  enables  us  to  regard  the  whole  story  of  organic  life, 
from  the  earliest  beginning  to  the  present  day,  as  a  vast 
mechanical  process.  It  is  reserved  for  phylogeny  "  to  reduce 
the  constructive  forces  of  the  animal  body  to  the  general 
forces  or  life-tendencies  of  the  universe."  No  sooner  does 
the  science  of  the  evolution  of  species  shed  its  light  on  the 
dark  puzzles  of  embryology  than  the  true  laws  of  develop- 
ment take  definite  shape. 

It  is  becoming  clearer  every  year  that  this  alone  is  the 
right  path;  that  the  facts  of  ontogeny  can  only  be  really 
explained  by  the  theories  of  phylogeny.  Moreover,  the 
number  and  importance  of  the  facts  which  we  borrow  from 
two  other  fields  of  research,  the  cognate  sciences  of  paleon- 
tology and  comparative  anatomy,  also  grow  every  year.  The 
profound  and  intimate  connection  of  the  historical  documents 
furnished  by  these  two  sciences  with  those  of  ontogeny  is 
growing  clearer  and  more  impressive  the  more  we  penetrate 
to  these  three  sources  of  history.  The  need  for  using  the 
three  classes  of  documents  in  equal  measure  and  with 
discrimination  in  the  tracing  of  our  ancestral  tree  is  more 
evident  every  day. 

These  leading  principles,  which  I  had  presented  and 
followed  in  the  first  edition  of  the  Anthropogenie,  have  been 
applied  far  more  thoroughly  and  comprehensively  in  the 
fourth  edition,  as  our  biological  knowledge  has  been  great* 
enlarged  in  all  three  fields  of  inquiry  during  the  last  fifteen 
years.  In  thus  recognising  and  appreciating  these  general 
biogenetic  principles,  I  find  myself  completely  opposed  to  the 


PREFACE  TO  THE  FOURTH  EDITION  <xk 

purely  descriptive  and  so-called  "exact"  method  of  embrvo- 
logical  study,  which  takes  the  careful  description  o(  the  facts 
of  the  science  to  be  its  sole  proper  purpose.  When  this 
"descriptive  embryology"  rises,  in  spite  of  its  restriction,  to 
an  explanation  of  the  facts  it  describes,  it  assumes  the  proud 
title  of  "physiological  embryology."  It  fancies  it  has  found 
the  real  mechanical  causes  of  the  facts  of  embryology  when 
it  has  traced  them  to  simple  physical  processes,  such  as  the 
bending  and  folding  of  elastic  plates,  the  hollowing  of 
vesicles,  and  so  forth. 

The  chief  defect  of  this  "exact"  or  physiological — it 
would  be  better  to  say,  "  pseudo-mechanical  " — method  in 
embryology  is  seen  in  its  attempt  to  reduce  most  complex 
historical  processes  to  simple  physical  phenomena.  When, 
for  instance,  the  spinal  cord  of  the  vertebrate  embryo  severs 
itself  from  the  general  envelope,  or  when  the  five  cerebral 
vesicles  are  formed  by  transverse  folds  at  its  bulbous  upper 
extremity,  it  might  seem  to  a  superficial  observer  that  these 
are  simple  physical  processes.  But  we  do  not  really  under- 
stand them  until  we  trace  them  to  their  true  phylogenetic 
causes,  and  see  that  each  of  these  apparently  simple  processes 
is  the  recapitulation  of  a  long  series  of  historical  changes 
(modified  by  being  inherited  in  a  concentrated  form),  for  the 
production  of  which  in  the  race-history  of  our  animal 
ancestors  a  vast  number  of  instances  of  adaptation  and 
heredity  have  co-operated  during  millions  of  years. 
Naturally,  each  of  these  physiological  processes  has  in  turn 
been  determined  by  mechanical  causes,  or  by  physical  and 
chemical  conditions  ;  but  these  are  far  removed  from  direct 
and  exact  observation,  as  they  are  "  pre-historic  "  phenomena 
of  the  remote  past. 

I  have  alreadv,  in  my  essays  on  Aims  and  Methods  <>/ 
the  Modern  Science  of  Evolution  (1875)  and  The  Origin  and 
Development  of  Animal  Tissues  (1884),  pointed  out  the  chief 
errors  of  this  pretentious  "  mechanical  science  of  embryology," 
and  shown  its  radical  opposition  to  our  phylogenetic  method. 
Surprise  has  often  been  expressed  that  so  superficial  a  method, 
directed  solely  to  the  external  appearance  of  the  embryonic 


PREFACE  TO  THE  FOURTH  EDITIOX 


processes,  and  ignoring  their  historic  nature,  should  have 
attained  such  considerable  results.  It  is  due  mainly  to  the 
restriction  of  its  aim.  This  narrowness  of  the  pseudo- 
mechanical  school  is,  in  fact,  three-fold.  Firstly,  it  restricts 
itself  in  the  use  of  its  empirical  material,  as  it  only  uses  one 
of  the  three  great  documents — ontogeny — and  ignores  the 
other  two — paleontology  and  comparative  anatomy.  Secondly, 
it  restricts  itself  in  its  scientific  method,  in  assuming  as  its 
sole  aim  the  exact  determination,  with  rule  and  compasses,  of 
the  embryonic  forms.  And,  thirdly,  it  restricts  itself  in  its 
philosophic  insight,  since  it  excludes  all  comparison  with 
cognate  phenomena  and  all  correlation  of  the  parts  with  the 
whole.  However,  this  concentration — in  itself  a  most  prolific 
source  of  error — is  welcomed  in  many  quarters  to-day,  at  a 
time  when  the  narrowest  specialism  obtains  its  greatest 
triumphs,  when  the  study  of  history  is  reversed,  and  when 
every  thoughtful  scientist  who  looks  to  the  connection  of 
phenomena  is  tabooed  as  "a  natural  philosopher."  For  all 
that,  the  scienc'e  of  evolution  is  an  historical,  and  not  an 
"  exact,"  inquiry. 

Convinced  that  this  method  of  anthropogenetic  research 
is  the  method  of  the  future,  I  conclude  with  the  hope  that 
this  enlarged  fourth  edition  of  the  Anthropogenic  may,  like  its 
predecessors,  contribute  towards  the  enkindling  of  a  deeper 
interest  in  the  most  important  basis  of  anthropology.  "  Know 
thyself":  that  is  the  source  of  all  wisdom.  But  it  is  impos- 
sible for  a  man  to  have  real  self-knowledge  unless  he  is 
acquainted  with  the  story  of  his  development. 

Ernst  Haeckel. 

Jena.  Angus/  18th,  iSiji. 


PREFACE   TO   THE    FIFTH    EDITION 


Nearly  thirty  years  have  elapsed  since  the  appearance  of 
the  first  edition  of  the  Anthrqpogente,  and  twelve  years  since 
the  publication  of  the  fourth  edition.  In  the  long  interval 
scientific  research  into  the  subject  of  the  work  lias  made 
extraordinary  progress,  not  only  in  the  great  enlargement  of 
the  field  of  inquiry  and  the  multiplication  of  workers,  but 
also  by  the  improvement  of  methods  and  greater  thorough- 
ness in  the  treatment  of  the  most  important  questions.  Hence 
I  found  it  no  light  task  to  undertake  a  new  issue  of  my  work 
after  such  a  lapse  of  time,  and  in  advanced  age.  But,  after 
long  hesitation,  I  was  moved  to  do  so  by  the  following  con- 
siderations. 

My  Anthropogenic  was  in  a  twofold  sense  a  "  first  attempt  " 
when  it  appeared  in  1874.  In  the  first  place,  I  approached 
the  difficult  and  hitherto  neglected  task  of  applying  to  man 
the  chief  law  of  biogeny  in  all  its  force,  and  of  giving  a 
hypothetical  sketch  of  the  course  of  his  ancestral  develop- 
ment founded  on  the  observed  facts  of  embryology.  But  I 
also  made  the  still  more  difficult  attempt  to  render  these  com- 
plicated embryological  facts,  and  the  cognate  theories  of 
phytogeny,  intelligible,  not  merely  to  the  small  circle  of 
my  scientific  colleagues,  but  also,  by  a  popular  presentation, 
to  the  general  public.  In  both  respects  my  work  lias  remained 
for  thirty  years  the  only  one  of  its  kind  ;  and  on  this  account 
I  deemed  it  my  duty,  in  spite  of  its  great  defects,  of  which  I 
am  not  unconscious,  to  undertake  a  revision  of  the  book. 

Many  disapproved  of  the  presentation  of  so  difficult  and 
delicate  a  subject  to  the  general  reader.  A  number  of  my 
colleagues  expressed  the  opinion  that  it  was  impossible  and 
undesirable  to  give  a  popular  treatment  of  so  obscure  and 
unfamiliar  a  study  as  human   embryology  ;  and   that   it   was 


PREFACE  TO  THE  FIFTH  EDITIOX 


still  more  regrettable  to  associate  with  these  facts  of  embryo- 
logy the  airy  and  precarious  hypotheses  of  phylogeny.  This 
academic  view,  which  is  widely  shared  in  learned  circles,  was 
extended  to  the  popularisation  of  the  whole  science  of  evolu- 
tion and  the  monistic  conception  "of  life  which  is  founded 
thereon.  I  have  never  been  able  to  accept  this  opinion  of 
the  German  professors  ;  I  share,  on  the  contrary,  the  view  of 
the  learned  among  our  neighbours,  that  the  whole  educated 
world  has  a  right  to  be  acquainted  with  the  most  important 
advances  of  science,  even  when  their  general  results  are  only 
matters  of  theory  and  are  opposed  to  the  prevailing  beliefs. 
It  is  enough  to  quote  the  instance  of  geology.  With  this 
conviction  I  undertook,  in  my  History  of  Creation,  in  1868, 
the  difficult  task  of  introducing  the  modern  science  of  evolu- 
tion, founded  bv  Darwin,  to  the  general  reader,  and  to  win 
for  phylogeny  the  general  recognition  which  its  sister-science, 
geology,  had  long  enjoyed.  The  immense  correspondence  I 
have  had  in  connection  with  the  ten  editions  of  this  book  has 
proved  to  me  that  it  met  a  real  want  on  the  part  of  the  public. 
The  same  may  be  said  of  my  work,  The  Riddle  of  the  Universe, 
in  which  I  gathered  together  the  conclusions  of  fifty  years  of 
study  in  1899.  I  attribute  the  remarkable  success  of  this 
"  popular  study  of  the  monistic  philosophy "  to  no  special 
merit  of  my  book,  but  to  the  eagerness  of  the  majority  of 
educated  people  to  acquaint  themselves  with  the  results  of 
progressive  science  and  cast  off  the  superstitions  of  conven- 
tional theology  and  metaphysics. 

Interest  in  the  embryology  of  plants  and  animals — that  is, 
in  the  experimental  study  of  these  mysterious  processes — has 
increased  during  the  last  ten  years  to  an  extent  that  was 
undreamt-of  fifty  years  ago.  Every  year  a  number  of 
specialist  publications  are  issued  which  deal  with  one  or 
other  subject  in  this  very  attractive  and  most  fruitful  field  of 
research.  An  introduction  to  this  wonderful  study,  once  so 
remote  and  exclusive,  is  provided  by  well-illustrated  manuals 
and  text-books.  Unfortunately,  many  of  these  works  show 
a  lack  of  general  morphological  (or  anatomical)  knowledge, 
and  of  the  indispensable  method  of  comparison  with  related 


PREFACE  TO  THE  FIFTH  EDITION  xxiii 

phenomena — not  only  of  "  comparative  embryology,"  but  also 
"comparative  anatomy ";  that  is  to  say,  of  a  discerning-  and 
philosophical  study  of  the  complicated  conditions  of  the 
whole  series  of  tonus,  or  the  stem,  to  which  the  organism  in 
question  belongs.  It  is  also  necessary  to  have  a  thorough 
preparatory  training  in  classification,  or  an  acquaintance  with 
the  relations  of  affinity,  on  the  ground  of  which  our  "natural 
system  "  arranges  the  classes,  orders,  families,  and  so  on.  I 
have  shown  in  my  Systematische  Phylogenie*  (1894-6 — three 
volumes)  how  profound  an  insight  this  "  phyletic  classifica- 
tion "  gives  us  into  the  history  of  the  stem. 

Paleontology  is  even  more  neglected  than  comparative 
anatomy  and  classification  by  most  of  our  modern  embrvo- 
logists.  Many  of  them  are  totally  ignorant  of  it.  Never- 
theless, the  fossils,  the  historical  succession  and  systematic 
arrangement  of  which  are  taught  in  paleontology,  are  just 
as  important  documents  for  the  history  ot  the  stem  as  the 
embryos  which  are  taken  by  these  one-sided  embryologists 
to  be  the  only  fitting  subject  of  research.  We  must,  it  is 
true,  grant  that  most  of  the  paleontologists  are  equally 
narrow  ;  they  commonly  lack  the  necessary  preliminary 
training  in  comparative  anatomv  and  embryology  which  is 
indispensable  for  the  correct  appreciation  of  the  fossilised 
remains  and  their  phvlogenetic  significance. 

It  was  my  chief  and  constant  care,  in  the  heavy  task  of 
preparing  this  fifth  edition  of  my  Anthropogenic,  to  avoid  this 
narrowness,  and  to  use  all  three  documents  bearing  on  our 
ancestral  history  in  even  greater  force  and  harmony  than  in 
the  preceding  editions.  Paleontology,  comparative  anatomy, 
and  ontogeny  must  complete  each  other's  work,  and  give  to 
the  historical  hypotheses  of  ph\  logeny  that  firmness  and 
fullness  which  they  are  bound  to  secure.  In  order  to  make 
this  work  accessible  to  a  wider  class  of  readers,  I  have 
considerably  increased  the  number  of  illustrations  in  the 
present  edition.  The  number  of  plates  (originally  twelve)  is 
now    thirty,    and     the     illustrations    in    the    text    have    been 

1  Not  translated  into  English. 


PREFACE  TO  THE  FIFTH  EDITIOX 


increased  from  210  to  512  ;  the  number  of  genealogical  tables 
is  raised  from  thirty-six  to  sixty.  The  text  has  also  been 
much  extended  ;  the  forty-six  sheets  of  the  first  edition,  and 
fifty-seven  of  the  fourth,  have  now  grown  to  sixty-two.  I 
have,  nevertheless,  left  unchanged  the  general  arrangement 
of  the  thirty  chapters.  I  must  express  my  gratitude  to  the 
house  of  Wilhelm  Engelmann  for  the  excellent  production  of 
the  work  and  assistance  in  preparing  its  many  illustrations  ; 
and  to  my  pupil,  Heinrich  Schmidt,  for  his  aid  in  correcting 
proofs  and  revision  of  the  index. 

To  speak  of  the  alterations  in  detail,  most  of  the  chapters 
have  been  substantially  improved,  and  some  of  them  have 
been  entirely  re-written.  I  thought  it  necessary  to  include  at 
least  the  most  important  advances  that  have  been  made  in 
each  branch  from  the  vast  and  increasing  literature  of  the 
subject.  I  fear  that  many  errors  may  have  been  overlooked. 
That  was  inevitable  in  view  of  the  intricacy  of  the  work  and 
the  defects  of  the  craftsman.  Yet  I  hope  the  book  will  attain 
its  chief  purpose  of  introducing  the  thoughtful  reader  into  the 
great  and  wonderful  realm  of  the  evolution  of  man,  and 
stimulate  him  to  reflect  on  its  significance.  I  would  include 
especially  teachers,  doctors,  and  students,  among  these 
"thoughtful  readers";  but  I  appeal  also  to  the  many 
educated  men  and  women  who  desire  to  know  the  full  truth 
as  to  the  origin  and  development  of  their  individual  being 
and  the  place  of  man  in  nature. 

Ernst  Haeckel. 

Jena,  September  yth,  igoj. 


CHAPTER  I. 

THE  FUNDAMENTAL  LAW  OF  ORGANIC 
EVOLUTION1 

General  importance  of  the  science  of  human  evolution.  Ignorance  of  it 
among  educated  people  Tin-  two  sections  of  the  science  of  evolution  : 
Ontogeny  or  embryology,  and  Phytogeny  or  stem-history.  Causal  connec- 
tion between  the  two  sections.  Phytogeny  is  the  cause  of  ontogeny. 
Ontogeny  as  a  summary  or  recapitulation  of  Phytogeny.  Incompleteness 
of  this  summary.  The  chief  law  of  biogeny.  Heredity  and  adaptation  are 
the  two  constructive  functions,  or  the  mechanical  causes,  of  evolution. 
Exclusion  of  final  causes.  Sole  validity  of  mechanical  causes.  Supplant- 
ing of  the  dualistic  by  the  monistic  philosophy.  Great  importance  of  tin' 
facts  of  embryology  for  the  monistic  philosophy.  Palingenesis  and 
cenogenesis.  Evolution  of  structure  and  function.  Necessary  connection 
ofphysiogeny  and  morphogeny.  Evolutionary  science  hitherto  an  achieve- 
ment of  morphology,  not  physiology.  The  evolution  of  the  central  nervous 
system  (the  brain  and  spinal  cord)  proceeds  step  by  step  with  that  of  the 
psychic  or  mental  life. 

The  field  of  natural  phenomena  into  which  I  would  introduce 
my  readers  in  the  following  chapters  has  a  quite  peculiar 
place  in  the  broad  realm  of  scientific  inquiry.  There  is  no 
object  of  investigation  that  touches  man  more  closely,  and 
the  knowledge  of  which  should  be  more  acceptable  to  him, 
than  his  own  frame.  But  among  all  the  various  branches  of 
the  natural  history  of  mankind,  or  anthropology,  the  story  of 
his  development  bv  natural  means  must  excite  the  most 
lively  interest.  It  gives  us  the  key  of  the  great  world- 
riddles  at  which  the  human  mind  has  been  working  for 
thousands  of  years.  The  problem  of  the  nature  of  man,  or 
the  question  of  man's  place  in  nature,  and  the  cognate 
inquiries  as  to  the  past,  the  earliest  history,  the  present 
situation,  and  the  future  of  humanity — all  these  most 
important  questions  are  directly  and  intimately  connected 
with  that  branch  of  study  which  we  call  the  science  of  the 

'  The  English  works  recommended  by  Professor  llaeekel  are  :  Chap.  xiii. 
of  Darwin's  Origin  of  Species,  Spencer's  Principles  »/'  Biology,  and  Haeckel's 
Riddle  of  the  Universe. — Trans. 


THE  FUXDAMEXTAI.  LA  W  OF  ORGAXIC  EVOLUTIOX 


evolution  of  man,  or,  in  one  word,  "  Anthropogeny  "  (the 
genesis  of  man).  Yet  it  is  an  astonishing  but  incontestable 
fact  that  the  science  of  the  evolution  of  man  does  not  even 
yet  form  part  of  the  scheme  of  general  education.  In  fact, 
educated  people  even  in  our  day  are  for  the  most  part  quite 
ignorant  of  the  important  truths  and  remarkable  phenomena 
which  anthropogeny  teaches  us. 

As  an  illustration  of  this  curious  state  of  things,  it  may  be 
pointed  out  that  most  of  what  are  considered  to  be  "  educated" 
people  do  not  know  that  every  human  being  is  developed 
from  an  egg,  or  ovum,  and  that  this  egg  is  one  simple  cell, 
like  any  other  plant  or  animal  egg.  They  are  equally 
ignorant  that  in  the  course  of  the  development  of  this  tiny, 
round  egg-cell  there  is  first  formed  a  body  that  is  totally 
different  from  the  human  frame,  and  has  not  the  remotest 
resemblance  to  it.  Most  of  them  have  never  seen  such  a 
human  foetus  or  embryo  in  the  earlier  period  of  its  develop- 
ment, and  do  not  know  that  it  is  quite  indistinguishable  from 
other  animal  embryos.  At  first  the  embryo  is  no  more  than 
a  globular  group  of  cells,  then  it  becomes  a  simple  hollow 
sphere,  the  wall  of  which  is  composed  of  a  layer  of  cells. 
Later  it  approaches  very  closely,  at  one  period,  to  the 
anatomic  structure  of  the  lancelot,  afterwards  to  that  of  a  fish, 
and  again  to  the  typical  build  of  the  amphibia  and  mammals. 
As  it  continues  to  develop  a  form  appears  which  is  like  those 
we  find  at  the  lowest  stage  of  mammal-life  (such  as  the  duck- 
bills), then  a  form  that  resembles  the  marsupials,  and  only  at 
a  late  stage  a  form  that  has  a  resemblance  to  the  ape;  until  at 
last  the  definite  human  form  emerges  and  closes  the  series  of 
transformations.  These  suggestive  facts  are,  as  I  said,  still 
almost  unknown  to  the  general  public — so  completely 
unknown  that,  if  one  casually  mentions  them,  they  are  called 
into  doubt  or  denied  outright  as  fairy-tales.  Everybody 
knows  that  the  butterfly  emerges  from  the  pupa,  and  the 
pupa  from  a  quite  different  thing  called  a  larva,  and  the 
larva  from  the  butterfly's  egg.  But  few  besides  medical  men 
are  aware  that  man,  in  the  course  of  his  individual  formation, 
passes  through  a  series  of  transformations  which  are  not  less 


THE  FUNDAMENTAL  I A  II'  OF  OKCAX/C  EVOLUTION 


surprising  and  wonderful  than  the  familiar  metamorphoses  of 
the  butterfly. 

The  mere  description  of  these  remarkable  changes  through 
which  man  passes  during  his  embryonic  life  should  arouse 
considerable  interest.  But  the  mind  will  experience  a  far 
keener  satisfaction  when  we  trace  these  curious  facts  to  their 
causes,  and  when  we  learn  to  behold  in  them  natural  pheno- 
mena which  are  of  the  highest  importance  throughout  the 
whole  field  of  human  knowledge.  The}'  throw  light  first  of 
all  on  the  "  natural  history  of  creation,"  then  on  psycho- 
logy, or  ••  the  science  of  the  soul,"  and  through  this  on  the 
whole  of  philosophy.  And  as  the  general  results  of  every 
branch  of  inquiry  are  summed  up  in  philosophy,  all  the 
sciences  come  in  turn  to  be  touched  and  influenced  more  or 
less  by  the  study  of  the  evolution  of  man. 

But  when  I  say  that  I  propose  to  present  here  the  most 
important  features  of  these  phenomena  and  trace  them  to 
their  causes,  I  take  the  term,  and  I  interpret  my  task,  in  a 
very  much  wider  sense  than  is  usual.  The  lectures  which 
have  been  delivered  on  this  subject  in  the  universities  during 
the  last  half-century  are  almost  exclusively  adapted  to  medical 
men.  Certainly,  the  medical  man  has  the  greatest  interest  in 
Studying  the  origin  of  the  human  body,  with  which  he  is 
daily  occupied.  But  I  must  not  give  here  this  special  descrip- 
tion of  the  embryonic  processes  such  as  it  has  hitherto  been 
given,  as  most  of  my  readers  have  not  studied  anatomy,  and 
are  not  likely  to  be  entrusted  with  the  care  of  the  adult 
organism.  1  must  content  myself  with  giving  some  parts  of 
the  subject  only  in  general  outline,  and  must  not  enter  upon 
all  the  marvellous,  but  very  intricate  and  not  easily  described, 
details  that  are  found  in  the  story  of  the  development  of  the 
human  frame.  To  understand  these  fully  a  knowledge  of 
anatomy  is  needed.  I  will  endeavour  to  be  as  plain  as  pos- 
sible in  dealing  with  this  branch  of  science.  Indeed,  a 
sufficient  general  idea  of  the  course  of  the  embryonic  develop- 
ment of  man  can  be  obtained  without  going  too  closely  into 
the  anatomic  details.  I  trust  we  may  be  able  to  arouse  the 
same   interest   in   this  delicate  field  oi    inquiry  as  has  been 


4  THE  FUNDAMENTAL  LA  W  OF  ORGANIC  EVOLUTION 

excited  already  in  other  branches  of  science  ;  though  we  shall 
meet  more  obstacles  here  than  elsewhere. 

The  story  of  the  evolution  of  man,  as  it  has  hitherto  been 
expounded  to  medical  students,  has  usually  been  confined  to 
embryology — or,  more  correctly,  ontogeny — or  the  science  of 
the  development  of  the  individual  human  organism.  But 
this  is  really  only  the  first  part  of  our  task,  the  first  half  of 
the  story  of  the  evolution  of  man  in  that  wider  sense  in  which 
we  understand  it  here.  We  must  add  as  the  second  half — as 
another  and  not  less  important  and  interesting  branch  of  the 
science  of  the  evolution  of  the  human  stem — phylogeny  : 
this  may  be  described  as  the  science  of  the  evolution  of  the 
various  animal  forms  from  which  the  human  organism  has 
been  developed  in  the  course  of  countless  ages.  Everybody 
now  knows  of  the  great  scientific  activity  that  was  occasioned 
by  the  publication  of  Darwin's  Origin  of  Species  in  1859. 
The  chief  direct  consequence  of  this  publication  was  to 
provoke  a  fresh  inquiry  into  the  origin  of  the  human  race, 
and  this  has  proved  beyond  question  our  gradual  evolution 
from  the  lower  species.  We  give  the  name  of  "  Phylogeny  " 
to  the  science  which  describes  this  ascent  of  man  from  the 
lower  ranks  of  the  animal  world.  The  chief  source  that  it 
draws  upon  for  facts  is  "Ontogeny,"  or  embryology,  the 
science  of  the  development  of  the  individual  organism.  More- 
over, it  derives  a  good  deal  of  support  from  paleontology,  or 
the  science  of  fossil  remains,  and  even  more  from  comparative 
anatomy,  or  morphology. 

These  two  branches  of  our  science — on  the  one  side 
ontogeny  or  embryology,  and  on  the  other  phylogeny,  or  the 
science  of  race-evolution — are  most  vitally  connected.  The 
one  cannot  be  understood  without  the  other.  It  is  only  when 
the  two  branches  fully  co-operate  and  supplement  each  other 
that  "  Biogeny  "  (or  the  science  of  the  genesis  of  life  in  the 
widest  sense)  attains  to  the  rank  of  a  philosophic  science. 
The  connection  between  them  is  not  external  and  superficial, 
but  profound,  intrinsic,  and  causal.  This  is  a  discovery  made 
by  recent  research,  and  it  is  most  clearly  and  correctly 
expressed    in  the  comprehensive    law   which    I   have   called 


THE  FUNDAMENTAL  LA  W  OF  ORGANIC  EVOLUTION  5 

"the  fundamental  law  o(  organic  evolution,"  or  "  the  funda- 
mental law  of  biogenv."  This  general  law,  to  which  we 
shall  find  ourselves  constantly  recurring,  and  on  the  recogni- 
tion of  which  depends  one's  whole  insight  into  the  story  of 
evolution,  may  be  briefly  expressed  in  the  phrase:  "The 
history  of  the  fcetus  is  a  recapitulation  of  the  history  of  the 
race";  or,  in  other  words,  "Ontogeny  is  a  recapitulation  o( 
phylogeny."  It  may  be  more  fully  stated  as  follows:  The 
series  of  forms  through  which  the  individual  organism  passes 
during  its  development  from  the  ovum  to  the  complete  bodily 
structure  is  a  brief,  condensed  repetition  of  the  long  scries 
of  forms  which  the  animal  ancestors  of  the  said  organism, 
or  the  ancestral  forms  of  the  species,  have  passed  through 
from  the  earliest  period  of  organic  life  down  to  the  present 
day. 

The  causal  character  of  the  relation  which  connects 
embrvology  with  stem-history  is  due  to  the  action  of  heredity 
and  adaptation.  When  we  have  rightly  understood  these, 
and  recognised  their  great  importance  in  the  formation  of 
organisms,  we  can  go  a  step  further  and  say:  Phylogenesis 
is  the  mechanical  cause  of  ontogenesis.1  In  other  words, 
the  development  of  the  stem,  or  race,  is  the  cause,  in  accor- 
dance with  the  phvsiological  laws  of  heredity  and  adaptation, 
of  all  the  changes  which  appear  in  a  condensed  form  in  the 
evolution  of  the  fcetus. 

The  chain  of  manifold  animal  forms  which  represent  the 
ancestrv  of  each  higher  organism,  or  even  of  man,  according 
to  the  theory  of  descent,  always  form  a  connected  whole. 
We  may  designate  this  uninterrupted  series  of  forms  with 
the  letters  of  the  alphabet:  A,  B,  C,  D,  E,  etc.,  to  Z.  In 
apparent  contradiction  to  what  I  have  said,  the  story  ot  the 
development  of  the  individual,  or  the  ontogeny  of  most 
organisms,  only  offers  to  the  observer  a  part  of  these  forms; 

1  The  term  "genesis,"  which  recurs  throughout,  means,  of  course,  "birth  " 
or  "origin.'1  From  this  we  get:  Biogeny=the  origin  of  life  (.bios)  \  Anthro- 
pogenj  :  the  origin  of  taan(anthropos)  ;  Ontogeny  the  origin  of  the  individual 
(  on  j;  Phylogeny = the  origin  of  the  species  (phulon  )  -,  and  so  on.  In  each  case 
the  1. -rin  may  refer  to  tin-  process  itself,  or  to  the  science  describing  the 
process. — Trans. 


THE  FVXDAMEXTAL  LA  W  OF  ORGAXIC  EYOLVTIOX 


so  that  the  defective  series  of  embryonic  forms  would  run: 
A,  B,  D,  F,  H,  K,  M,  etc.;  or,  in  other  cases,  B,  D,  H,  L, 
M,  N,  etc.  Here,  then,  as  a  rule,  several  of  the  evolutionary- 
forms  of  the  original  series  have  fallen  out.  Moreover,  we 
often  find — to  continue  with  our  illustration  from  the  alphabet 
— one  or  other  of  the  original  letters  of  the  ancestral  series 
represented  by  corresponding  letters  from  a  different  alphabet. 
Thus,  instead  of  the  Roman  B  and  D,  we  often  have  the 
Greek  B  and  A.  In  this  case  the  text  of  the  biogenetic  law 
has  been  corrupted,  just  as  it  had  been  abbreviated  in  the 
preceding  case.  But,  in  spite  of  all  this,  the  series  of 
ancestral  forms  remains  the  same,  and  we  are  in  a  position  to 
discover  its  original  complexion. 

In  reality,  there  is  always  a  certain  parallel  between  the 
two  evolutionary  series.  But  it  is  obscured  from  the  fact 
that  in  the  embryonic  succession  much  is  wanting  that 
certainly  existed  in  the  earlier  ancestral  succession.  If  the 
parallel  of  the  two  series  were  complete,  and  if  this  great 
fundamental  law  affirming  the  causal  nexus  between  ontogeny 
and  phylogeny  in  the  proper  sense  of  the  word  were  directly 
demonstrable,  we  should  only  have  to  determine,  by  means 
of  the  microscope  and  the  dissecting  knife,  the  series  of  forms 
through  which  the  fertilised  ovum  passes  in  its  development; 
we  should  then  have  before  us  a  complete  picture  of  the 
remarkable  series  of  forms  which  our  animal  ancestors  have 
successively  assumed  from  the  dawn  of  organic  life  down  to 
the  appearance  of  man.  But  such  a  repetition  of  the 
ancestral  history  by  the  individual  in  its  embryonic  life  is 
very  rarely  complete.  We  do  not  often  find  our  full  alphabet. 
In  most  cases  the  correspondence  is  very  imperfect,  being 
greatly  distorted  and  falsified  by  causes  which  we  will  con- 
sider later.  We  are  thus,  for  the  most  part,  unable  to 
determine  in  detail,  from  the  study  of  its  embryology,  all  the 
different  shapes  which  an  organism's  ancestors  have  pre- 
sented ;  we  usually — and  especially  in  the  case  of  the  human 
foetus — encounter  many  gaps.  It  is  true  that  we  can  fill  up 
most  of  these  gaps  satisfactorily  with  the  help  of  comparative 
anatomy,   but    we    cannot  do  so    from    direct    embryological 


THE  FUNDAMENTAL  LA  II'  OF  ORGANIC  EVOLUTION  7 

observation.  Hence  it  is  important  that  we  find  a  large 
number  of  lower  animal  forms  to  be  still  represented  in  the 
course  of  man's  embryonic  development.  In  these  cases  we 
may  draw  our  conclusions  with  the  utmost  security  as  to  the 
nature  of  the  ancestral  form  from  the  features  of  the  form 
which  the  embryo  momentarily  assumes. 

To  give  a  few  examples,  we  can  infer  from  the  fact  that 
the  human  ovum  is  a  simple  cell  that  the  first  ancestor  of  our 
species  was  a  tiny  unicellular  being,  something  like  the 
amoeba.  In  the  same  way,  we  know,  from  the  fact  that  the 
human  foetus  consists,  at  the  first,  of  two  simple  cell-layers 
(the  gastrula J,  that  the  gas trcea,  a  form  with  two  such  layers, 
was  certainly  in  the  line  of  our  ancestry.  A  later  human 
embryonic  form  (the  chordula)  points  just  as  clearly  to  a 
worm-like  ancestor  (the  prochordouici J,  the  nearest  living 
relation  of  which  is  found  among  the  actual  ascidia.  To 
this  succeeds  a  most  important  embrvonic  stage  ( ' acnuiia J, 
in  which  our  headless  fcetus  presents,  in  the  main,  the 
structure  of  the  amphioxus.  But  we  can  only  indirectly  and 
approximatelv,  with  the  aid  of  comparative  anatomy  and 
ontogeny,  conjecture  what  lower  forms  enter  into  the  chain 
of  our  ancestry  between  the  gastrasa  and  the  chordula,  and 
between  this  and  the  amphioxus.  In  the  course  of  the 
historical  development  (by  means  of  heredity  in  a  condensed 
form)  many  intermediate  structures  have  gradually  fallen  out, 
which  must  certainly  have  been  represented  in  our  ancestry. 
But,  in  spite  of  these  many,  and  sometimes  very  appreciable, 
gaps,  there  is  no  contradiction  between  the  two  successions. 
In  fact,  it  is  the  chief  purpose  of  this  work  to  prove  the  real 
harmony  and  the  original  parallelism  of  the  two.  I  hope  to 
show,  on  a  substantial  basis  of  facts,  that  we  can  draw  most 
important  conclusions  as  to  our  genealogical  tree  from  the 
actual  and  easily-demonstrable  series  of  embryonic  changes. 
We  shall  then  be  in  a  position  to  form  a  general  idea  of  the 
wealth  of  animal  forms  which  have  figured  in  the  direct  line 
of  our  ancestry  in  the  lengthy  history  of  organic  life. 

In  this  phylogenetic  appreciation  of  the  facts  of  embryo- 
logy we  must,  of  course,  take  particular  care  to  distinguish 


8  THE  FUNDAMENTAL  LA  W  OF  ORGAXIC  EVOLUTION 

sharply  and  clearly  between  the  primitive,  palingenetic  (or 
ancestral)  evolutionary  processes  and  those  due  to  ceno- 
genesis. '  By  palingenetic  processes,  or  embryonic  recapitu- 
lations, we  understand  all  those  phenomena  in  the  development 
of  the  individual  which  are  transmitted  from  one  generation 
to  another  by  heredity,  and  which,  on  that  account,  allow  us 
to  draw  direct  inferences  as  to  corresponding  structures  in  the 
development  of  the  species.  On  the  other  hand,  we  give  the 
name  of  cenogenetic  processes,  or  embryonic  variations,  to  all 
those  phenomena  in  the  foetal  development  that  cannot  be 
traced  to  inheritance  from  earlier  species,  but  are  due  to  the 
adaptation  of  the  foetus,  or  the  infant-form,  to  certain  condi- 
tions of  its  embryonic  development.  These  cenogenetic 
phenomena  are  foreign  or  later  additions ;  they  allow  us  to  draw 
no  direct  inference  whatever  as  to  corresponding  processes 
in  our  ancestral  history,  but  rather  hinder  us  from  doing  so. 

This  careful  discrimination  between  the  primary  or 
palingenetic  processes  and  the  secondary  or  cenogenetic  is 
of  great  importance  for  the  purposes  of  the  scientific  history 
of  a  species,  which  has  to  draw  conclusions  from  the  available 
facts  of  embryology,  comparative  anatomy,  and  paleontology, 
as  to  the  processes  in  the  formation  of  the  species  in  the 
remote  past.  It  is  of  the  same  importance  to  the  student  of 
evolution  as  the  careful  distinction  between  genuine  and 
spurious  texts  in  the  works  of  an  ancient  writer,  or  the 
purging  of  the  real  text  from  interpolations  and  alterations,  is 
for  the  student  of  philology.  It  is  true  that  this  distinction 
has  not  yet  been  fully  appreciated  by  many  scientists.  For 
my  part,  I  regard  it  as  the  first  condition  for  forming  any  just 
idea  of  the  evolutionary  process,  and  I  believe  that  we  must, 
in  accordance  with  it,  divide  embryology  into  two  sections — 
palingenesis,  or  the  science  of  repetitive  forms  ;  and  ceno- 
genesis,  or  the  science  of  supervening  structures. 

'  Palingenesis  =  new  birth,  or  re-incarnation  (palin  =  again,  genesis  or 
genea  =  development)  ;  hence  its  application  to  the  phenomena  which  are 
recapitulated  by  heredity  from  earlier  ancestral  forms.  Cenogenesis  =  foreign 
or  negligible  development  (kenos  and  genea)  ;  hence,  those  phenomena  which 
come  later  in  the  story  of  life  to  disturb  the  inherited  structure,  by  a  fresh 
adaptation  to  environment. — Trans. 


THE  FUNDAMENTAL  LAW  OF  ORGANIC  EVOLUTION 


To  give  at  once  a  few  examples  from  the  science  of 
man's  origin  in  illustration  of  this  important  distinction,  I 
may  instance  the  following  processes  in  the  embryology  oi 
man,  and  of  all  the  higher  vertebrates,  as  palingenetic :  the 
formation  of  the  two  primary  germinal  layers  and  of  the 
primitive  gut,  the  undivided  structure  of  the  dorsal  nerve- 
tube,  the  appearance  of  a  simple  axial  rod  between  the 
medullary  tube  and  the  gut,  the  temporary  formation  of  the 
gill-clefts  and  arches,  the  primitive  kidneys,  and  so  on.  All 
these,  and  many  other  important  structures,  have  clearly 
been  transmitted  by  a  steady  heredity  from  the  early 
ancestors  of  the  mammal,  and  are,  therefore,  direct  indica- 
tions of  the  presence  of  similar  structures  in  the  history  of 
the  stem.  On  the  other  hand,  this  is  certainly  not  the  case 
with  the  following  embryonic  changes,  which  we  must 
describe  as  cenogenetic  processes  :  the  formation  of  the 
yelk-sac,  the  allantois,  the  placenta,  the  amnion,  the 
serolemma,  and  the  chorion — or,  generally  speaking,  the 
various  foetal  membranes  and  the  corresponding  changes 
in  the  blood  vessels.  Further  instances  are  :  the  dual 
structure  of  the  heart  cavity,  the  temporary  division  of  the 
plates  of  the  primitive  vertebra?  and  lateral  plates,  the 
secondary  closing  of  the  ventral  and  intestinal  walls,  the 
formation  of  the  navel,  and  so  on.  All  these  and  many 
other  phenomena  are  certainly  not  traceable  to  similar 
structures  in  any  earlier  and  completely-developed  ancestral 
form,  but  have  arisen  simply  by  adaptation  to  the  peculiar 
conditions  of  embryonic  life  (within  the  fcetal  membranes). 
In  view  of  these  facts,  we  may  now  give  the  following  more 
precise  expression  to  our  chief  law  of  hiogeny  : — The 
evolution  of  the  foetus  (or  ontogenesis)  is  a  condensed  and 
abbreviated  recapitulation  of  the  evolution  of  the  stem  (or 
phylogenesis)  ;  and  this  recapitulation  is  the  more  complete  in 
proportion  as  the  original  development  (or  palingenesis)  is 
preserved  by  a  constant  heredity  ;  on  the  other  hand,  it 
becomes  less  complete  in  proportion  as  a  varying  adaptation 
to  new  conditions  increases  the  disturbing  factors  in  the 
development  (or  catagenesis). 


io  THE  FUNDAMENTAL  LA  W  OF  ORGANIC  EVOLUTION 

The  cenogenetic  alterations  or  distortions  of  the  original 
palingenetic  course  of  development  take  the  form,  as  a  rule, 
of  a  gradual  displacement  of  the  phenomena,  which  is  slowly 
effected  by  adaptation  to  the  changed  conditions  of  embryonic 
existence  during  the  course  of  thousands  of  years.  This 
displacement  may  take  place  as  regards  either  the  locality  or 
the  time  of  a  phenomenon.  The  first  is  called  heterotopism, 
the  second  heterochronism. 

Heterotopisms,  or  variations  in  locality,  affect,  in  the  first 
place,  the  cells,  or  elementary  parts  of  which  the  organs  are 
composed  ;  but  they  also  affect  the  organs  themselves. 
Thus,  for  instance,  the  sexual  glands  in  the  human  embryo, 
and  most  of  the  higher  animals,  arise  out  of  the  middle 
germinal  layer.  On  the  other  hand,  the  comparative  embry- 
ology of  the  lower  animals  shows  us  that  originally  they  did 
not  arise  from  this,  but  from  one  of  the  primary  germinal 
layers.  However,  the  germ-cells  have  gradually  changed 
their  position,  and  passed  over  at  so  early  a  period  from  their 
original  situation  into  the  middle  layer  that  they  now  seem 
really  to  arise  from  it.  A  similar  heterotopism  is  observed  in 
the  case  of  the  primitive  renal  (kidney)  passages  of  the  higher 
vertebrates,  which  originally  took  their  rise  in  the  external 
skin.  Even  in  the  case  of  the  origin  of  the  mesoderm 
(middle-skin)  itself  heterotopism,  in  connection  with  a 
removal  of  embryonic  cells  from  one  skin  layer  to  another, 
plays  an  important  part. 

Heterochronism,  or  variation  in  time,  is  not  less  instruc- 
tive. It  consists  in  the  fact  that  the  series  of  forms  in  which 
the  organs  successively  appear  is  different  in  embryology 
from  what  the  stem  history  leads  us  to  expect.  Just  as  the 
spatial  disposition  is  falsified  in  heterotopism,  so  we  find 
the  time  arrangement  altered  in  heterochronism.  This  may 
appear  either  as  an  acceleration  or  a  delay  in  the  rise  of 
an  organ.  As  cases  of  ontogenetic  acceleration  we  may 
instance,  in  the  embryonic  development  of  man,  the  early 
appearance  of  the  heart,  the  gill-clefts,  the  brain,  the  eyes, 
etc.  These  organs  clearly  arise  much  earlier,  in  comparison 
with  others,  than  was  originally  the  case  with  our  ancestors. 


THE  FUNDAMENTAL  /../  II'  OF  ORGANIC  EVOLUTION 

We  find  the  reverse  of  this  in  the  retarded  formation  of  the 
gut,  the  ventral  cavity,  and  the  sexual  organs.  These  are 
clear  instances  of  ontogenetic  retardation. 

The  great  importance  and  strict  regularity  of  these  time 
variations  in  embryology  have  been  carefully  studied  recently 
by  Ernest  Mehnert,  in  his  Biomcchanik  (Jena,  1898).  He 
formulates  his  "chief  law  of  organogenesis"  in  the  following 
words  :  "  The  rapiditv  of  the  embryonic  development  of  an 
organ  is  in  proportion  to  its  stage  of  evolution,  which  has 
been  retarded  for  a  time.  It  rises  with  the  increase  and  falls 
with  the  diminution  of  the  stage  of  evolution  once  attained." 
Mehnert  contends  that  our  biogenetic  law  has  not  been 
impaired  by  the  attacks  of  its  opponents,  and  goes  on  to  say : 
"  Scarcelv  any  piece  of  knowledge  has  contributed  so  much  to 
the  advance  of  embryology  as  this  ;  its  formulation  is  one  of 
the  most  signal  services  to  general  biology.  It  was  not  until 
this  law  passed  into  the  flesh  and  blood  of  investigators,  and 
they  had  accustomed  themselves  to  see  a  reminiscence  of 
ancestral  history  in  embryonic  structures,  that  we  witnessed 
the  great  progress  which  embryological  research  has  made 
in  the  last  two  decades."  The  best  proof  of  the  correctness  of 
this  opinion  is  that  now  the  most  fruitful  work  is  done  in  all 
branches  of  embryology  with  the  aid  of  this  biogenetic  law, 
and  that  it  enables  students  to  attain  every  year  thousands 
of  brilliant  results  that  they  would  never  have  reached 
without  it. 

It  is  only  when  one  appreciates  the  cenogenetic  processes 
in  relation  to  the  palingenetic,  and  when  one  takes  careful 
account  of  the  changes  which  the  latter  may  suffer  from  the 
former,  that  the  radical  importance  of  the  biogenetic  law  is 
recognised,  and  it  is  felt  to  be  the  most  illuminating  principle 
in  the  science  of  evolution.  In  this  task  of  discrimination  it 
is  the  silver  thread  in  relation  to  which  we  can  arrange  all 
the  phenomena  of  this  realm  of  marvels — the  "Ariadne 
thread,"  which  alone  enables  us  to  find  our  way  through 
this  labyrinth  of  forms.  Hence  the  brothers  Sarasin,  the 
zoologists,  could  say  with  perfect  justice,  in  their  study  of  the 
evolution  of  the  Ichthyopliis,  that  "  the  great  biogenetic  law  is 


THE  FUXDAMEXTAL  LA  IV  OF  ORGAXIC  EVOLUTIOX 


just  as  important  for  the  zoologist  in  tracing  long-extinct 
processes  as  spectrum  analysis  is  for  the  astronomer." 

Even  at  an  earlier  period,  when  a  correct  acquaintance 
with  the  evolution  of  the  human  and  animal  frame  was  only 
just  being  obtained — and  that  is  scarcely  eighty  years  ago! — 
the  greatest  astonishment  was  felt  at  the  remarkable  similarity 
observed  between  the  embryonic  forms,  or  stages  of  foetal 
development,  in  very  different  animals  ;  attention  was  called 
even  then  to  their  close  resemblance  to  certain  fully-developed 
animal  forms  belonging  to  some  of  the  lower  groups.  The 
older  scientists  (Oken,  Treviranus,  and  others)  knew  perfectly 
well  that  these  lower  forms  in  a  sense  illustrated  and  fixed,  in 
the  hierarchy  of  the  animal  world,  a  temporary  stage  in  the 
evolution  of  higher  forms.  The  famous  anatomist  Meckel 
spoke  in  1821  of  a  "similarity  between  the  development  of 
the  embryo  and  the  series  of  animals."  Baer  raised  the 
question  in  1828  how  far,  within  the  vertebrate  type,  the 
embryonic  forms  of  the  higher  animals  assume  the  permanent 
shapes  of  members  of  lower  groups.  But  it  was  impossible 
fully  to  understand  and  appreciate  this  remarkable  resem- 
blance at  that  time.  We  owe  our  capacity  to  do  this  to  the 
theory  of  descent;  it  is  this  that  puts  in  their  true  light  the 
action  of  heredity  on  the  one  hand  and  adaptation  on  the 
other.  It  explains  to  us  the  vital  importance  of  their  constant 
reciprocal  action  in  the  production  of  organic  forms.  Darwin 
was  the  first  to  teach  us  the  great  part  that  was  played  in  this 
by  the  ceaseless  struggle  for  existence  between  living  things, 
and  to  show  how,  under  the  influence  of  this  (by  natural 
selection),  new  species  were  produced  and  maintained  solely 
by  the  interaction  of  heredity  and  adaptation.  It  was  thus 
Darwinism  that  first  opened  our  eyes  to  a  true  comprehension 
of  the  supremely  important  relations  between  the  two  parts  of 
the  science  of  organic  evolution — Ontogeny  and  Phylogeny. 

Heredity  and  adaptation  are,  in  fact,  the  two  constructive 
physiological  functions  of  living  things :  unless  we  understand 
these  properly  we  can  make  no  headway  in  the  study  of 
evolution.  Hence,  until  the  time  of  Darwin  no  one  had 
a   clear   idea   of   the    real    nature   and    causes   of  embryonic 


THE  FUNDAMENTAL  /..I  II'  OF  ORGANIC  EVOLUTION  13 

development.  It  was  impossible  to  explain  the  curious  series 
of  forms  through  which  the  human  embryo  passed ;  it  was 
quite  unintelligible  why  this  strange  succession  of  animal-like 
forms  appeared  in  the  series  at  all.  It  had  previously  been 
generally  assumed  that  the  man  was  found  complete  in  all 
his  parts  in  the  ovum,  and  that  the  development  consisted 
only  in  an  unfolding  of  the  various  parts,  a  simple  process  of 
growth.  This  is  by  no  means  the  case.  On  the  contrary, 
the  whole  process  of  the  development  of  the  individual 
presents  to  the  observer  a  connected  succession  of  various 
animal-forms  ;  and  these  forms  display  a  great  variety  of 
external  and  internal  structure.  But  why  each  individual 
human  being  should  pass  through  this  series  of  forms  in  the 
course  of  his  embryonic  development  it  was  quite  impossible 
to  say  until  Lamarck  and  Darwin  established  the  theory  of 
descent.  Through  this  theory  we  have  at  last  detected  the 
real  causes,  the  causce  efficientes,  of  the  individual  develop- 
ment; we  have  learned  that  these  mechanical  causes  suffice  of 
themselves  to  effect  the  formation  of  the  organism,  and  that 
there  is  no  need  of  the  final  causes  which  were  formerly 
assumed.  It  is  true  that  in  the  academic  philosophies  of  our 
time  these  final  causes  still  figure  very  prominently;  in  the 
new  philosophy  of  nature  we  can  entirely  replace  them  by 
efficient  causes. 

Before  I  pass  from  the  subject  I  must  speak  further  of  this, 
one  of  the  most  brilliant  achievements  of  the  human  mind 
in  modern  times.  The  history  of  philosophy  shows  us  that 
final  causes  are  still  generally  regarded  in  philosophic  circles, 
just  as  among  the  philosophers  of  antiquity,  as  the  real 
sources  of  the  phenomena  of  organic  life,  and  especially  o\ 
human  life.  This  dominant  teleology,  which  is  largely  based 
on  Kant,  assumes  that  the  processes  of  organic  life,  especially 
those  of  development,  can  only  be  explained  by  final  causes, 
and  are  not  susceptible  of  a  mechanical — that  is  to  say,  a 
really  scientific — explanation.  But  the  darkest  enigmas  which 
had  hitherto  beset  us  in  this  connection,  and  which  seemed  to 
be  only  approachable  through  teleology,  have  been  fully 
solved  in  a  mechanical  sense  by  the  theory  of  descent.     The 


i4  THE  FUNDAMENTAL  LA  W  OF  ORGANIC  EVOLUTION 

reconstruction  of  the  science  of  human  evolution  which  this 
brought  about  removed  the  greatest  impediments  from  the 
path  of  research.  We  shall  see,  in  the  course  of  our  inquiry, 
how  the  most  wonderful  and  hitherto  insoluble  enigmas  in 
the  human  and  animal  frame  have  proved  amenable  to  a 
mechanical  explanation,  by  causes  acting  without  prevision, 
through  Darwin's  reform  of  the  science  of  evolution.  We 
have  everywhere  been  able  to  substitute  unconscious  causes, 
acting  from  necessity,  for  conscious,  purposive  causes.1 

If  the  new  science  of  evolution  had  done  no  more  than 
this,  every  thoughtful  man  would  have  to  admit  that  it  had 
accomplished  an  immense  advance  in  knowledge.  It  means 
that  in  the  whole  of  philosophy  that  tendency  which  we  call 
monistic,  in  opposition  to  the  dualistic,  which  has  hitherto 
prevailed,  must  be  accepted.2  At  this  point  the  science  of 
human  evolution  has  a  direct  and  profound  bearing  on  the 
foundations  of  philosophy.  I  have  dealt  with  this  relation 
very  fully  in  my  Riddle  of  the  Universe.  In  the  first  part  I 
show  how  modern  anthropology  has,  by  its  astounding  dis- 
coveries during  the  second  half  of  the  nineteenth  century, 
compelled  us  to  take  a  completely  monistic  view  of  life.  Our 
bodily  structure  and  its  life,  our  embryonic  development  and 
our  evolution  as  a  species,  teach  us  that  the  same  laws  of 
nature  rule  in  the  life  of  man  as  in  the  rest  of  the  universe. 
For  this  reason,  if  for  no  others,  it  is  desirable,  nay,  indispen- 
sable, that  every  man  who  wishes  to  form  a  serious  and  philo- 
sophic view  of  life,  and,  above  all,  the  expert  philosopher, 
should  acquaint  himself  with  the  chief  facts  of  this  branch  of 
science. 

1  The  monistic  or  mechanical  philosophy  of  nature  holds  that  only  uncon- 
scious, necessary,  efficient  causes  are  at  work  in  the  whole  field  of  nature,  in 
organic  life  as  well  as  in  inorganic  changes.  On  the  other  hand,  the  dualist 
or  vitalist  philosophy  of  nature  affirms  that  unconscious  forces  arc  only  at  work 
in  the  inorganic  world,  and  that  we  find  conscious,  purposive,  or  final  causes 
in  organic  nature. 

2  Monism  is  neither  purely  materialistic  nor  purely  spiritualistic,  but  a 
reconciliation  of  these  two  principles,  since  it  regards  the  whole  of  nature  as 
one,  and  sees  only  efficient  causes  at  work  in  it.  Dualism,  on  the  contrary, 
holds  that  nature  and  spirit,  matter  and  force,  the  world  and  God,  inorganic 
and  organic  nature,  are  separate  and  independent  existences.  Cf.  The  Riddle 
of  the  Universe,  chap.  xii. 


THE  FUNDAMENTAL  LA  II'  OF  ORGANIC  EVOLUTIOX 


The  facts  of  embryology  have  so  great  and  obvious  a 
significance  in  this  connection  that  even  in  recent  years 
dualist  and  teleological  philosophers  have  tried  to  rid  them- 
selves of  them  by  simply  denying  them.  This  was  done,  for 
instance,  as  regards  the  fact  that  man  is  developed  from  an 
egg,  and  that  this  egg  or  ovum  is  a  simple  cell,  as  in  the  case 
of  other  animals.  When  I  had  explained  this  pregnant  fact 
and  its  significance  in  my  Natural  History  of  Creation,  it 
was  described  in  many  of  the  theological  journals  as  a 
dishonest  invention  of  my  own.  The  fact  that  the  embryos 
of  man  and  the  dog  are,  at  a  certain  stage  of  their  develop- 
ment, almost  indistinguishable,  was  also  denied.  When  we 
examine  the  human  embryo  in  the  third  or  fourth  week  of  its 
development,  we  find  it  to  be  quite  different  in  shape  and 
structure  from  the  full-grown  human  being,  but  almost 
identical  with  that  of  the  ape,  the  dog,  the  hare,  and  other 
mammals,  at  the  same  stage  of  ontogeny.  We  find  a  bean- 
shaped  body  of  very  simple  construction,  with  a  tail  below 
and  a  pair  of  fins  at  the  sides,  something  like  those  of  a  fish, 
but  very  different  from  the  limbs  of  man  and  the  mammals. 
Nearly  the  whole  front  half  of  the  body  is  taken  up  by  a 
shapeless  head  without  face,  at  the  sides  of  which  we  find 
gill-clefts  and  arches  as  in  the  fish  (see  the  thirteenth  plate  at 
the  end  of  Chapter  xiv.).  At  this  stage  of  its  development 
the  human  embryo  does  not  differ  in  any  essential  detail  from 
that  of  the  ape,  dog,  horse,  ox,  etc.,  at  a  corresponding 
period.  This  important  fact  can  easily  be  verified  at  any 
moment  by  a  comparison  of  the  embryos  of  man,  the  dog, 
hare,  etc.  Nevertheless,  the  theologians  and  dualist  philo- 
sophers pronounced  it  to  be  a  materialistic  invention ;  even 
scientists,  to  whom  the  facts  should  be  known,  have  sought 
to  denv  them. 

There  could  not  be  a  clearer  proof  of  the  profound 
importance  of  these  embrvcilogical  facts  in  favour  of  the 
monistic  philosophy  than  is  afforded  by  these  efforts  of  its 
opponents  to  get  rid  of  them  by  silence  or  denial.  The  truth 
is  that  these  facts  are  most  inconvenient  for  them,  and  are 
quite   irreconcilable   with    their   views.      We  must  be  all   the 


16  THE  FUNDAMENTAL  LA  W  OF  ORGANLC  EVOLUTION 

more  pressing  on  our  side  to  put  them  in  their  proper  light. 
I  fully  agree  with  Huxley  when  he  says,  in  his  Man's  Place 
in  Nature :  "  Though  these  facts  are  ignored  by  several  well- 
known  popular  leaders,  they  are  easy  to  prove,  and  are 
accepted  by  all  scientific  men ;  on  the  other  hand,  their 
importance  is  so  great  that  those  who  have  once  mastered 
them  will,  in  my  opinion,  find  few  other  biological  discoveries 
to  astonish  them." 

We  shall  make  it  our  chief  task  to  study  the  evolution  of 
man's  bodily  frame  and  its  various  organs  in  their  external 
form  and  internal  structures.  But  I  may  observe  at  once 
that  this  is  accompanied  step  by  step  with  a  study  of  the 
evolution  of  their  functions.  These  two  branches  of  inquiry 
are  inseparably  united  in  the  whole  of  anthropology,  just  as 
in  zoology  (of  which  the  former  is  only  a  section)  or  general 
biology.  Everywhere  the  peculiar  form  of  the  organism  and 
its  structures,  internal  and  external,  is  directly  related  to  the 
special  physiological  functions  which  the  organism  or  organ 
has  to  execute.  This  intimate  connection  of  structure  and 
function,  or  of  the  instrument  and  the  work  done  by  it,  is 
seen  in  the  science  of  evolution  and  all  its  parts.  Hence  the 
story  of  the  evolution  of  structures,  which  is  our  immediate 
concern,  is  also  the  history  of  the  development  of  functions; 
and  this  holds  good  of  the  human  organism  as  of  anv  other. 

At  the  same  time,  I  must  admit  that  our  knowledge  of  the 
evolution  of  functions  is  very  far  from  being  as  complete  as 
our  acquaintance  with  the  evolution  of  structures.  One 
might  say,  in  fact,  that  the  whole  science  of  evolution,  or 
biogeny  (both  in  ontogeny  and  phylogeny),  has  almost 
confined  itself  to  the  study  of  structures  ;  the  biogeny  of 
functions  hardly  exists  even  in  name.  That  is  the  fault  of 
the  physiologists,  who  have  as  yet  concerned  themselves  very 
little  about  evolution.  It  is  only  in  recent  times  that  physio- 
logists like  W.  Engelmann,  W.  Preyer,  M.  Verworn,  and 
a  few  others,  have  attacked  the  biogeny  of  functions. 

For  a  long  time  now  the  two  great  branches  of  biological 
research,  morphology  and  physiology,  have  pursued  separate 
ways.     That  is  quite  natural.     The  aims  and  methods  of  the 


THE  FUNDAMENTAL  LA  W  OF  ORGANIC  EVOLUTIOX  17 

two  are  very  different.  Morphology  (anatomy),  or  the  science 
of  forms,  seeks  a  scientific  knowledge  of  organic  struc- 
ture, internal  and  external.  On  the  other  hand,  physiology, 
or  the  science  of  functions,  studies  the  vital  phenomena. 
The  two  together  make  up  biology.  But  the  development  of 
physiology  during  the  last  fifty  years  has  been  much  more  one- 
sided than  that  of  morphology.  It  has  not  only  failed  to  make 
much  use  of  the  comparative  method,  which  has  given  such 
great  results  in  morphology,  but  it  has  also  neglected  evolu- 
tionarv  principles.  Hence  in  the  last  few  decades  morpho- 
logy has  far  outrun  physiology,  though  the  latter  is  apt  to 
put  on  superior  airs  in  regard  to  its  rival.  Morphology  has 
achieved  its  finest  results  in  the  way  of  comparative  anatomy 
and  ontogeny,  and  nearly  all  that  I  shall  put  before  the 
reader  in  this  work  as  to  the  evolution  of  man  has  been 
obtained  by  the  labours,  not  of  the  physiologist,  but  of  the 
morphologist.  In  fact,  the  one-sidedness  of  modern  physio- 
logy  is  so  great  that  it  has  hitherto  neglected  the  study  of  the 
most  important  evolutionary  functions,  heredity  and  adapta- 
tion, and  abandoned  even  these  purely  physiological  subjects 
to  the  morphologist.  We  owe  nearly  all  that  we  know  about 
them  to  the  morphologist,  not  to  the  physiologist.  The  latter 
concerns  himself  little  more  with  the  functions  (or  agencies) 
of  evolution  than  with  the  evolution  of  functions. 

It  will  be  the  task  of  some  future  physiologist  to  engage 
in  the  study  of  the  evolution  of  functions  with  the  same  zeal 
and  success  as  has  been  done  for  the  evolution  of  structures 
in  morphogeny  (the  genesis  of  forms).  Let  me  illustrate  the 
close  connection  of  the  two  by  a  couple  of  examples.  The 
heart  in  the  human  embryo  has  at  first  a  very  simple  con- 
struction, such  as  we  find  in  permanent  form  among  the 
ascidia  and  other  low  organisms;  with  this  is  associated  a 
very  simple  system  of  circulation  of  the  blood.  Now,  when 
we  find  that  with  the  full-grown  heart  there  comes  a  totally 
different  and  much  more  intricate  circulation,  our  inquiry 
into  the  development  of  the  heart  becomes  at  once,  not  only  a 
morphological,  but  also  a  physiological,  study.  Thus  it  is 
clear  that  the  ontogeny  of  the  heart  can  only  be  understood  in 


THE  FVXDAMEXTAL  LA  W  OF  ORGA.XIC  EVOLUTIOX 


the  light  of  its  phylogeny  (or  development  in  the  past),  both 
as  regards  function  and  structure.  The  same  holds  true  of 
all  the  other  organs  and  their  functions.  For  instance,  the 
science  of  the  evolution  of  the  alimentary  canal,  the  lungs,  or 
the  sexual  organs,  gives  us  at  the  same  time,  through  the 
exact  comparative  investigation  of  structure-development, 
most  important  information  with  regard  to  the  evolution  of 
the  functions  of  these  organs. 

This  significant  connection  is  very  clearlv  seen  in  the 
evolution  of  the  nervous  system.  This  system  is  in  the 
economy  of  the  human  body  the  medium  of  sensation,  will, 
and  even  thought,  the  highest  of  the  psychic  functions  ;  in  a 
word,  of  all  the  various  functions  which  constitute  the  proper 
object  of  psychology.  Modern  anatomy  and  physiology  have 
proved  that  these  psychic  functions  are  immediately  dependent 
on  the  fine  structure  and  the  composition  of  the  central 
nervous  system,  or  the  internal  texture  of  the  brain  and 
spinal  cord.  In  these  we  find  the  elaborate  cell-machinery, 
of  which  the  psychic  or  soul-life  is  the  physiological  function. 
It  is  so  intricate  that  most  men  still  look  upon  the  mind 
as  something  supernatural  that  cannot  be  explained  on 
mechanical  principles. 

But  embryological  research  into  the  gradual  appearance 
and  the  formation  of  this  important  system  of  organs  yields 
the  most  astounding  and  significant  results.  The  first 
sketch  of  a  central  nervous  system  in  the  human  embryo 
presents  the  same  very  simple  type  as  in  the  other  vertebrates. 
A  spinal  tube  is  formed  in  the  external  skin  of  the  back,  and 
from  this  first  comes  a  simple  spinal  cord  without  brain,  such 
as  we  find  to  be  the  permanent  psychic  organ  in  the  lowest 
type  of  mammal,  the  amphioxus.  Not  until  a  later  stage  is  a 
brain  formed  at  the  anterior  end  of  this  cord,  and  then  it  is 
a  brain  of  the  most  rudimentary  kind,  such  as  we  find 
permanently  among  the  lower  fishes.  This  simple  brain 
developes  step  by  step,  successively  assuming  forms  which 
correspond  to  those  of  the  amphibia,  the  reptiles,  the  duck- 
bills, and  the  prosimias.  Only  in  the  last  stage  does  it 
reach   the    highly   organised    form    which    distinguishes   the 


THE  FUNDAMENTAL  LAW  OF  ORGANIC  EVOLUTION 

apes  from  the   other   vertebrates,  and  which    attains    its  full 
development  in  man. 

Comparative  physiology  discovers  a  precisely  similar 
growth.  The  function  of  the  brain,  the  psychic  activity,  rises 
step  bv  step  with  the  advancing  development  of  its  structure. 

Thus  we  are  enabled,  by  this  story  of  the  evolution  of  the 
nervous  system,  to  understand  at  length  the  natural  develop- 
ment of  the  human  mind  and  its  gradual  unfolding.  It  is 
onlv  with  the  aid  of  embryology  that  we  can  grasp  how  these 
highest  and  mjst  striking  faculties  of  the  animal  organism 
have  been  historicallv  evolved.  In  other  words,  a  knowledge 
of  the  evolution  of  the  spinal  cord  and  brain  in  the  human 
embryo  leads  us  directly  to  a  comprehension  of  the  historic 
development  (or  phylogenv)  of  the  human  mind,  that  highest 
of  all  faculties,  which  we  regard  as  something  so  marvellous 
and  supernatural  in  the  adult  man.  This  is  certainly  one  of 
the  greatest  and  most  pregnant  results  of  evolutionary  science. 
Happilv,  our  embrvological  knowledge  of  man's  central 
nervous  svstem  is  now  so  adequate,  and  agrees  so  thoroughly 
with  the  complementary  results  of  comparative  anatomy  and 
phvsiologv,  that  we  are  thus  enabled  to  obtain  a  clear  insight 
into  one  of  the  highest  problems  of  philosophy,  the  phy- 
logenv of  the  soul,  or  the  ancestral  history  of  the  mind  of 
man.  Our  chief  support  in  this  comes  from  the  embryo- 
logical  study  of  it,  or  the  ontogeny  of  the  soul.  This 
important  section  of  psychology  owes  its  origin  especially  to 
W.  Prever,  in  his  interesting  works,  The  Mind  of  the  Child 
(English  translation)  and  Spezielle  Physio/ogie  des  Embryo. 
The  Biography  of  a  Baby  (1900),  of  Milicent  Washburn 
Shinn,  also  deserves  mention.  |See  also  Preyer's  Mental 
Development  in  the  Child  (translation),  and  Sully's  Studies  of 
Childhood  and  Children's  Ways. ] 

In  this  way  we  follow  the  only  path  along  which  we  may 
hope  to  reach  the  solution  of  this  difficult  problem. 

Thirty-six  years  have  now  elapsed  since  I  established 
phylogeny  as  an  independent  science  and  showed  its  intimate 
causal  connection  with  ontogeny  in  my  (ienere/le  Morphologie  ; 
thirty  years  have  passed  since   I   gave  in  my  gastraea-theory 


THE  FUNDAMENTAL  LAW  OF  ORGANIC  EVOLUTION 


the  proof  of  the  justice  of  this,  and  completed  it  with  the 
theory  of  germinal  layers.  When  we  look  back  on  this 
period  we  may  ask,  What  has  been  accomplished  during  it  by 
the  fundamental  law  of  biogeny?  If  we  are  impartial,  we 
must  reply  that  it  has  proved  its  fertility  in  hundreds  of  sound 
results,  and  that  by  its  aid  we  have  acquired  a  vast  fund  of 
knowledge  which  we  should  never  have  obtained  without  it. 

There  has  been  no  dearth  of  attacks — often  violent 
attacks — on  my  conception  of  an  intimate  causal  connection 
between  ontogenesis  and  phylogenesis  ;  but  no  other  satis- 
factory explanation  of  these  important  phenomena  has  yet 
been  offered  to  us.  I  say  this  especially  with  regard  to 
Wilhelm  His's  theory  of  a  "mechanical  evolution,"  which 
questions  the  validity  of  phylogeny  generally,  and  would 
explain  the  complicated  embryonic  processes  without  going 
beyond  by  simple  physical  changes — such  as  the  bending  and 
folding  of  leaves  by  electricity,  the  origin  of  cavities  through 
unequal  strain  of  the  tissues,  the  formation  of  processes  by 
uneven  growth,  and  so  on.  But  the  fact  is  that  these 
embryological  phenomena  themselves  demand  explanation 
in  turn,  and  this  can  only  be  found,  as  a  rule,  in  the 
corresponding  changes  in  the  long  ancestral  series,  or  in  the 
physiological  functions  of  hereditv  and  adaptation. 

Heinrich  Schmidt  (of  Jena)  has  given  a  good  account  and 
criticism  of  the  many  attacks  on  the  biogenetic  law  in  his 
interesting  pamphlet,  Haeckel's  biogenetisches  Gmndgesetz 
und  seine  Gegner  (Odenkirchen,  1902).  He  shows  that  not 
only  distinguished  zoologists,  but  botanists  also,  have  recog- 
nised it,  and  made  profitable  use  of  it ;  it  holds  good  of  the 
evolution  of  plants  no  less  than  of  animals.  On  the  other 
hand,  none  of  its  critics  has  offered  anything  better  to  replace 
it.  Many  of  the  criticisms,  in  fact,  arise  from  pure  mis- 
understanding, as  is  quite  to  be  expected  in  so  difficult  and 
complicated  a  subject,  or  from  a  wrong  idea  of  the  relation  of 
cenogenesis  and  palingenesis.  But,  in  spite  of  all  this,  our 
knowledge  of  the  mutual  relations  of  these  two  series  of 
phenomena  grows  every  day,  and  our  conviction  increases 
that  "  Phylogenesis  is  the  mechanical  cause  of  ontogenesis." 


CHAPTER  II. 

THE   OLDER    EMBRYOLOGY 

Aristotle's  Generation  of  Animals.  His  acquaintance  with  the  embryology  of 
lower  animals.  Arrest  of  scientific  research  during  the  Middle  Ages.  The 
ris,-  of  embryology  at  the  beginning  of  the  seventeenth  century.  Fabricius 
ab  Aquapendente.  Harvey.  Marcello  Malpighi.  The  significance  of 
the  hatched  egg.  The  theory  of  Pre-formation  and  Scatulation  (Evolution 
and  Pre-delineation).  The  unfolding  of  parts  already  formed.  The  theory 
of  Scatulation  for  male  and  female.  Either  the  spermatozoon  or  the  egg  is 
the  pre-formed  individual.  Animaleulists  or  Spermatists  (Leeuwenhock, 
Hartsoeker,  Spallanzani).  Ovulists  (Haller,  Leibnitz,  Bonnet).  A  calcula- 
tion of  the  germs  stored  in  Eve's  ovary.  Discovery  of  parthenogenesis  by 
Bonnet.  Victory  of  the  Pre-formation  theory  owing-  to  the  authority  of 
Haller  and  Leibnitz.  Caspar  Friedrieh  Wolff.  His  life  and  works.  The 
theoria generationis.  New  formation,  or  epigenesis.  The  evolution  of  the 
alimentary  canal.  First  beginnings  of  the  theory  of  germinal  layers.  The 
metamorphosis  of  plants.  Germs  of  the  cell  theory.  Wolff's  monistic 
philosophy. 

It  is  in  many  ways  useful,  on  entering  upon  the  study  of  any 
science,  to  cast  a  glance  at  its  historical  development.  The 
saying  that  "everything  is  best  understood  in  its  growth" 
has  a  distinct  application  to  science.  While  we  follow  its 
gradual  development  we  get  a  clearer  insight  into  its  aims 
and  objects.  Moreover,  we  shall  see  that  the  present 
condition  of  the  science  of  human  evolution,  with  all  its 
characteristics,  can  only  be  rightly  understood  when  we 
examine  its  historical  growth.  This  task  will,  however,  not 
detain  us  long.  The  study  of  man's  evolution  is  one  of  the 
latest  branches  of  natural  science,  whether  you  consider  the 
embryological  or  the  phylogenetic  section  of  it. 

Apart  from  the  few  germs  of  our  science  which  we  find  in 
classical  antiquity,  and  which  we  shall  notice  presently,  we 
may  say  that  it  takes  its  definite  rise,  as  a  science,  in  the  yeat 
1 759,  when  one  of  the  greatest  German  scientists,  Caspar 
Friedrieh  Wolff,  published  his  Theoria  generationis.  That 
was  the  foundation-stone  of  the  science  of  animal  embryology. 
It  was  not  until  fifty  years  later,  in  1809,  that  Jean  Lamarck 


THE  OLDER  EMBRYOLOGY 


published  his  Pliilosophie  Zoologique — the  first  effort  to 
provide  a  base  for  the  theory  of  evolution  ;  and  it  was  another 
half-century  before  Darwin's  work  appeared  (in  1859),  which 
we  may  regard  as  the  first  scientific  attainment  of  this  aim. 
But  before  we  go  further  into  this  solid  establishment  of 
evolution,  we  must  cast  a  brief  glance  at  that  famous 
philosopher  and  scientist  of  antiquity,  who  stood  alone  in  this, 
as  in  many  other  branches  of  science,  for  more  than  2,000 
years  :  the  "  father  of  natural  history,"  Aristotle. 

The  extant  scientific  works  of  Aristotle  deal  with  many 
different  sides  of  biological  research ;  the  most  comprehensive 
of  them  is  his  famous  History  of  Animals.  But  not  less 
interesting  is  the  smaller  work,  On  the  Generation  of 
Animals  {Peri  zoon  geneseos).  This  work  treats  especially 
of  embryonic  development,  and  it  is  of  great  interest  as 
being  the  earliest  of  its  kind  and  the  only  one  that  has  come 
down  to  us  in  any  completeness  from  classical  antiquity. 
Like  Aristotle's  other  scientific  writings,  this  substantial  little 
work  has  dominated  the  whole  of  science  for  2,000  years. 
The  philosopher  was  as  keen  in  observation  as  he  was 
profound  in  thought.  Nevertheless,  while  his  philosophic 
distinction  has  never  been  questioned,  it  is  only  in  recent 
years  that  his  worth  as  an  observer  has  been  properly 
appreciated.  The  men  of  science  who  turned  to  his  scientific 
writings  about  the  middle  of  the  nineteenth  century  were 
astonished  at  the  amount  of  information  and  the  notable 
discoveries  that  they  found. 

In  connection  with  embryological  questions,  we  must 
particularly  note  that  Aristotle  studied  them  in  various 
classes  of  animals,  and  that  among  the  lower  groups  he 
learned  many  most  remarkable  facts  which  we  only  re-dis- 
covered between  1830  and  i860.  It  is  certain,  for  instance, 
that  he  was  acquainted  with  the  very  peculiar  mode  of 
propagation  of  the  cuttle-fishes,  or  cephalopods,  in  which  a 
yelk-sac  hangs  out  of  the  mouth  of  the  foetus.  He  knew, 
also,  that  embryos  come  from  the  eggs  of  the  bee  even  when 
they  have  not  been  fertilised.  This  "  parthenogenesis  "  (or 
virgin-birth)  of  the  bees  has  only  been  established    in   our 


THE  O/.OKh'  EMJ1KYOLOGY 


time   by   the   distinguished    zoologist    of   Munich,    Siebold. 

He  discovered  that  male  bees  come  from  the  unfertilised,  and 
female  bees  only  from  the  fertilised,  eggs.  Aristotle  further 
states  that  some  kinds  of  fishes  (of  the  genus  serranus)  are 
hermaphrodites,  each  individual  having  both  male  and  female 
organs  and  being  able  to  fertilise  itself ;  this,  also,  has  been 
recently  confirmed.  He  knew  that  the  embryo  of  many 
fishes  of  the  shark  family  is  attached  to  the  mother's  body  by 
a  sort  of  placenta,  or  nutritive  organ  very  rich  in  blood; 
apart  from  these,  such  an  arrangement  is  only  found  among 
the  higher  mammals  and  man.  This  placenta  of  the  shark 
was  looked  upon  as  legendary  for  a  long  time,  until  Johannes 
Miiller  proved  it  to  be  a  fact  in  18^9.  Thus  a  number  of 
remarkable  discoveries  were  found  in  Aristotle's  embryological 
work,  proving  a  very  good  acquaintance  of  the  great  scientist 
— possibly  helped  by  his  predecessors — with  the  facts  of 
ontogeny,  and  a  great  advance  upon  succeeding  generations 
in  this  respect. 

In  the  case  of  most  of  these  discoveries  he  did  not  merely 
describe  the  fact,  but  added  a  number  of  observations  on  its 
significance.  Some  of  these  theoretical  remarks  are  of  par- 
ticular interest,  because  they  show  a  correct  appreciation  of 
the  nature  of  the  embryonic  processes.  He  conceives  the 
development  of  the  individual  as  a  new  formation,  in  the 
course  of  which  the  various  parts  of  the  body  take  shape 
successively.  When  the  human  or  animal  frame  is  developed 
in  the  mother's  body,  or  separately  in  an  egg,  the  heart — 
which  he  regards  as  the  starting-point  and  centre  of  the 
organism — must  appear  first.  Once  the  heart  is  formed  the 
other  organs  arise,  the  internal  ones  before  the  external,  the 
upper  (those  above  the  diaphragm)  before  the  lower  (or  those 
beneath  the  diaphragm).  The  brain  is  formed  at  an  early 
stage,  and  the  eyes  grow  out  of  it.  These  observations  are 
quite  correct.  And,  if  we  try  to  form  some  idea  from  these 
data  of  Aristotle's  general  conception  of  the  embryonic 
process,  we  find  a  dim  prevision  of  the  theory  which  we  now 
call  epigenestSf  and  which  Wolff  showed  2,000  years  after- 
wards to  be  the  correct  view.      It  is  significant,  for  instance, 


THE  OLDER  EMBRYOLOGY 


that  Aristotle  denied  the  eternity  of  the  individual  in  any 
respect.  He  said  that  the  species  or  genus,  the  group  of 
similar  individuals,  might  be  eternal,  but  the  individual  itself 
is  temporary.  It  comes  into  being  in  the  act  of  procreation, 
and  passes  away  at  death. 

During  the  2,000  years  after  Aristotle  no  progress  what- 
ever was  made  in  general  zoology,  or  in  embryology  in 
particular.  People  were  content  to  read,  copy,  translate,  and 
comment  on  Aristotle.  Scarcely  a  single  independent  effort 
at  research  was  made  in  the  whole  of  the  period.  During 
the  Middle  Ages  the  spread  of  strong  religious  beliefs  put 
formidable  obstacles  in  the  way  of  independent  scientific 
investigation.  There  was  no  question  of  resuming  the 
advance  of  biology.  Even  when  human  anatomy  began  to 
stir  itself  once  more  in  the  sixteenth  century,  and  independent 
research  was  resumed  into  the  structure  of  the  developed 
body,  anatomists  did  not  dare  to  extend  their  inquiries  to  the 
unformed  body,  the  embryo,  and  its  development.  There 
were  many  reasons  for  the  prevailing  horror  of  such  studies. 
It  is  natural  enough,  when  we  remember  that  a  Bull  of 
Boniface  VIII.  excommunicated  every  man  who  ventured  to 
dissect  a  human  corpse.  If  the  dissection  of  a  developed 
body  were  a  crime  to  be  thus  punished,  how  much  more 
dreadful  must  it  have  seemed  to  deal  with  the  embryonic  body 
still  enclosed  in  the  womb,  which  the  Creator  himself  had 
decently  veiled  from  the  curiosity  of  the  scientist !  The 
Christian  Church,  then  putting  many  thousands  to  death  for 
unbelief,  had  a  shrewd  presentiment  of  the  menace  that 
science  contained  against  its  authority.  It  was  powerful 
enough  to  see  that  its  rival  did  not  grow  too  quickly. 

It  was  not  until  the  Reformation  broke  the  power  of  the 
Church,  and  a  refreshing  breath  of  the  spirit  dissolved  the 
icy  chains  that  bound  science,  that  anatomy  and  embryology, 
and  all  the  other  branches  of  research,  could  begin  to 
advance  once  more.  However,  embryology  lagged  far 
behind  anatomy.  The  first  works  on  embryology  appear 
at  the  beginning  of  the  sixteenth  century.  The  Italian 
anatomist,  Fabricius  ab  Aquapendente,  a  professor  at  Padua, 


Efc»Xrs?:r<* 


THE  OLDER  EMBRYOLOGY 


opened  the  advance.  In  his  two  books  [De  formato  foetu, 
1600,  and  De  format  iane  fee  I  us,  1604)  he  published  the  older 
illustrations  and  descriptions  of  the  embryos  of  man  and 
other  mammals,  and  of  the  hen.  Similar  imperfect  illustra- 
tions were  given  by  Spigelius  (De  formato  foetu,  1 631),  and 
by  Needham  (1667)  and  his  more  famous  compatriot,  Harvey 
(1652),  who  discovered  the  circulation  of  the  blood  in  the 
animal  body  and  formulated  the  important  principle,  Omne 
vivum  ex  vivo  (all  life  comes  from  pre-existing  life).  The 
Dutch  scientist,  Swammerdam,  published  in  his  Bible  of 
Nature  the  earliest  observations  on  the  embryology  of  the 
frog  and  the  division  of  its  egg-yelk.  But  the  most 
important  embryological  studies  in  the  sixteenth  century 
were  those  of  the  famous  Italian,  Marcello  Malpighi,  of 
Bologna,  who  led  the  way  both  in  zoology  and  botany.  His 
treatises,  De  formatione  pulli  and  De  ovo  incubato  (1687), 
contain  the  first  consistent  description  of  the  development  of 
the  chick  in  the  fertilised  egg. 

Here  I  ought  to  say  a  word  about  the  important  part 
plaved  by  the  chick  in  the  growth  of  our  science.  The 
development  of  the  chick,  like  that  of  the  young  of  all  other 
birds,  agrees  in  all  its  main  features  with  that  of  the  other 
chief  vertebrates,  and  even  of  man.  The  three  highest 
classes  of  vertebrates — mammals,  birds,  and  reptiles  (lizards, 
serpents,  tortoises,  etc.) — have  from  the  beginning  of  their 
embryonic  development  so  striking  a  resemblance  in  all  the 
chief  points  of  structure,  and  especially  in  their  first  forms, 
that  for  a  long  time  it  is  impossible  to  distinguish  between 
them  (see  plates  viii-xiii.).  We  have  known  now  for  some 
time  that  we  need  only  examine  the  embryo  of  a  bird,  which 
is  the  easiest  to  get  at,  in  order  to  learn  the  typical  mode  of 
development  of  a  mammal  (and  therefore  of  man).  As  soon 
as  scientists  began  to  study  the  human  embryo,  or  the 
mammal-embryo  generally,  in  its  earlier  stages  about  the 
middle  and  end  of  the  seventeenth  century,  this  important 
fact  was  very  quickly  discovered.  It  is  both  theoretically  and 
practically  of  great  value.  As  regards  the  theory  of  evolu- 
tion,  we   can    draw  the    most   weighty  inferences   from    this 


THE  OLDER  EMBRYOLOGY 


similarity  between  the  embryos  of  widely  different  classes  of 
animals.  But  for  the  practical  purposes  of  embryological 
research  the  discovery  is  invaluable,  because  we  can  fill  up 
the  gaps  in  our  imperfect  knowledge  of  the  embryology  of 
the  mammals  from  the  more  thoroughly  studied  embryology 
of  the  bird.  Hens'  eggs  are  easily  to  be  had  in  any  quantity, 
and  the  development  of  the  chick  may  be  followed  step  by 
step  in  artificial  incubation.  The  development  of  the 
mammal  is  much  more  difficult  to  follow,  because  here  the 
embryo  is  not  detached  and  enclosed  in  a  large  egg,  but  the 
tiny  ovum  remains  in  the  womb  until  the  growth  is  com- 
pleted. Hence,  it  is  very  difficult  to  keep  up  sustained 
observation  of  the  various  stages  in  any  great  extent,  quite 
apart  from  such  extrinsic  considerations  as  the  cost,  the 
technical  difficulties,  and  many  other  obstacles  which  we 
encounter  when  we  would  make  an  extensive  study  of  the 
fertilised  mammal.  The  chicken  has,  therefore,  always  been 
the  chief  object  of  study  in  this  connection.  The  excellent 
incubators  we  now  have  enable  us  to  observe  it  in  any 
quantity  and  at  any  stage  of  development,  and  so  follow  the 
whole  course  of  its  formation  step  by  step. 

By  the  end  of  the  seventeenth  century  Malpighi  had 
advanced  as  far  as  it  was  possible  to  do  with  the  imperfect 
microscope  of  his  time  in  the  embryological  study  of  the 
chick.  Further  progress  was  arrested  until  the  instrument 
and  the  technical  methods  should  be  improved.  The 
vertebrate  embryos  are  so  small  and  delicate  in  their  earlier 
stages  that  you  cannot  go  very  far  into  the  study  of  them 
without  a  good  microscope  and  other  technical  aid.  But 
this  substantial  improvement  of  the  microscope  and  the  other 
apparatus  did  not  take  place  until  the  beginning  of  the 
nineteenth  century. 

Embryology  made  scarcely  any  advance  in  the  first  half 
of  the  eighteenth  century,  when  the  systematic  natural  history 
of  plants  and  animals  received  so  great  an  impulse  through 
the  publication  of  Linne's  famous  Systema  Naturae.  Not 
until  1759  did  the  genius  arise  who  was  to  give  it  an  entirely 
new  character,  Caspar  Friedrich  Wolff.   Until  then  embryology 


THE  OLDER  EM  11  MYOLOGY  27 


had  been  occupied  almost  exclusively  in  unfortunate  and 
misleading  efforts  to  build  up  theories  on  the  imperfect 
empirical  material  then  available. 

The  theory  which  then  prevailed,  and  remained  in  favour 
throughout  nearly  the  whole  of  the  eighteenth  century,  was 
commonly  called  at  that  time  "the  evolution  theory";  it  is 
better  to  describe  it  as  "the  preformation  theory."1  Its 
chief  point  is  this  :  There  is  no  new  formation  of  structures  in 
the  embryonic  development  of  any  organism,  animal  or 
plant,  or  even  of  man  ;  there  is  only  a  growth,  or  unfolding, 
of  parts  which  have  been  constructed  and  ready  from  all 
eternity,  though  on  a  very  small  scale  and  closely  packed 
together.  Hence,  every  living  germ  contains  all  the  organs 
and  parts  of  the  bodv,  in  the  form  and  arrangement 
they  will  present  later,  already  within  it,  and  thus  the  whole 
embryological  process  is  merely  an  evolution  in  the  literal 
sense  of  the  word,  or  an  unfolding,  o{~  parts  that  were 
pre-formed  and  folded  up  in  it.  So,  for  instance,  we  find  in 
the  hen's  egg  not  merely  a  simple  cell,  that  divides  and  sub- 
divides and  forms  germinal  layers,  and  at  last,  after  all  kinds 
of  variation  and  cleavage  and  reconstruction,  brings  forth  the 
body  of  the  chick  ;  but  there  is  in  every  egg  from  the  first  a 
complete  chicken,  with  all  its  parts  made  and  neatly  packed. 
These  parts  are  so  small  or  so  transparent  that  the  microscope 
cannot  detect  them.  In  the  hatching,  these  parts  merely 
grow  larger,  and  spread  out  in  the  normal  way. 

When  this  theory  is  consistently  developed  it  becomes  a 
"scatulation  theory."-  According  to  its  teaching,  there  was 
made  in  the  beginning  one  couple  or  one  individual  of  each 
species  of  animal  or  plant  ;  but  this  one  individual  contained 
the  germs  of  all  the  other  individuals  of  the  same  species  who 
should    ever   come   to    life.     As   the   age   of    the    earth   was 

'This  theory  is  usually  known  as  the  "evolution  theory"  in  Germany,  in 
contradistinction  to  the  "epigenesis  theory."  Hut  as  it  is  the  latter  that  is 
called  the  "evolution  theorj  in  England,  France,  and  Italy,  and  "evolution" 
and  "epigenesis"  are  taken  to  be  synonymous,  ii  s,-c-ms  better  to  call  the  first 
the  "  preformation  theory."  Kolliker  has  recently  given  the  name  of  "evolu- 
tionism "  to  his  ••  theory  of  heterogeneous  conception." 

Packing  theory"  would  be  the  literal  translation.     Scatula  is  the  Latin 
for  a  case  or  box.— Trans. 


THE  OLDER  EMBRYOLOGY 


generally  believed  at  that  time  to  be  fixed  by  the  Bible  at 
5,000  or  6,000  years,  it  seemed  possible  to  calculate  how 
many  individuals  of  each  species  had  lived  in  the  period,  and 
so  had  been  packed  inside  the  first  being  that  was  created. 
The  theory  was  consistently  extended  to  man,  and  it  was 
affirmed  that  our  common  parent  Eve  had  had  stored  in  her 
ovary  the  germs  of  all  the  children  of  men. 

The  theory  at  first  took  the  form  of  a  belief  that  it  was  the 
females  who  were  thus  encased  in  the  first  being.  One 
couple  of  each  species  was  created,  but  the  female  contained 
in  her  ovary  all  the  future  individuals  of  the  species,  of  either 
sex.  However,  this  had  to  be  altered  when  the  Dutch 
microscopist,  Leeuwenhoek,  discovered  the  male  spermatozoa 
in  1690,  and  showed  that  an  immense  number  of  these 
extremely  fine  and  mobile  thread-like  beings  exist  in  the  male 
sperm  (this  will  be  explained  in  the  seventh  chapter).  This 
astonishing  discovery  was  further  advanced  when  it  was 
proved  that  these  living  bodies,  swimming  about  in  the 
seminal  fluid,  were  real  animalcules,  and,  in  fact,  were  the  pre- 
formed germs  of  the  future  generation.  When  the  male  and 
female  procreative  elements  came  together  at  conception, 
these  thread-like  spermatozoa  ("seed-animals")  were  supposed 
to  penetrate  into  the  fertile  body  of  the  ovum  and  begin  to 
develop  there,  as  the  plant  seed  does  in  the  fruitful  earth. 
Hence,  every  spermatozoon  was  regarded  as  a  homunculus,  a 
tiny  complete  man  ;  all  the  parts  were  believed  to  be  pre- 
formed in  it,  and  merely  grew  larger  when  it  reached  its 
proper  medium  in  the  female  ovum.  This  theory,  also,  was 
consistently  developed  in  the  sense  that  in  each  of  these 
thread-like  bodies  the  whole  of  its  posterity  was  supposed  to 
be  present  in  the  minutest  form.  Adam's  sexual  glands 
were  thought  to  have  contained  the  germs  of  the  whole  of 
humanity. 

This  "  theory  of  male  scatulation  "  found  itself  at  once  in 
keen  opposition  to  the  prevailing  "  female  "  theory.  All  that 
was  common  to  them  was  the  erroneous  idea  that  there  are  in 
every  germ  the  germs  of  innumerable  organisms  to  come 
enfolded  in  it — an  idea  that  served  as  the  ground  of  Linne's 


THE  OLDER  EMBRYOLOGY 


curious  "  prolepsis  theory."  The  two  rival  theories  at  once 
opened  a  very  lively  campaign,  and  the  physiologists  of  the 
eighteenth  century  were  divided  into  two  great  camps — the 
Animalculists  and  the  Ovulists — which  fought  vigorously. 
The  struggle  rather  amuses  us  to-day  when  we  know  that 
both  parties  were  wrong.  As  Kirchhoff  says  in  his  admir- 
able biographical  sketch  of  Wolff:  "This  controversy  was  as 
difficult  to  close  as  that  on  the  question  whether  the  angels 
live  in  the  eastern  or  the  western  part  of  heaven." 

The  animalculists  held  that  the  spermatozoa  were  the  true 
germs,  and  appealed  to  the  lively  movements  and  the 
structure  of  these  bodies.  In  the  case  of  man  and  most  of 
the  other  animals,  these  spermatozoa  have  a  rather  oval  or 
pear-shaped  head  and  a  thickish  stem,  ending  in  an  extremely 
fine  and  hair-like  tail  (Fig.  20).  The  whole  structure  is  really 
only  one  cell — a  ciliated  cell.  The  head  is  the  nucleus  en- 
closed in  a  little  of  the  cell-matter,  and  this  is  prolonged  in 
the  thick  stem  and  fine,  mobile  tail;  the  latter  is  the  "  whip  " 
(or  cilium)  by  which  it  moves  about,  and  corresponds  to  the 
cilium  in  a  ciliated  cell.  But  the  animalculists  believed  that 
the  "  head  "  was  a  real  head,  and  the  rest  of  it  a  complete 
body.  Leeuwenhoek,  Hartsoeker,  and  Spallanzani  were  the 
chief  champions  of  these  fantastic  speculations. 

The  opposing  party  of  the  Ovulists,  who  clung  to  the 
older  "  evolution  theory,"  affirmed  that  the  ovum  is  the  real 
germ,  and  that  the  spermatozoa  merely  stimulate  it  at  con- 
ception to  begin  its  growth;  all  the  future  generations  are 
stored  in  the  ovum.  This  view  was  held  by  the  great 
majority  of  the  biologists  of  the  eighteenth  century,  in  spite 
of  the  fact  that  Wolff  proved  it  in  1759  to  be  without  founda- 
tion. It  owed  its  prestige  chiefly  to  the  circumstance  that  the 
most  weighty  authorities  in  the  biology  and  philosophy  of 
the  day  decided  in  favour  of  it,  especially  Haller,  Bonnet, 
and  Leibnitz. 

Albrecht  Haller,  professor  at  Gbttingen,  who  is  often 
called  the  father  of  physiology,  was  a  man  of  wide  and  varied 
learning,  but  he  does  not  occupy  a  very  high  position 
in    regard    to     insight    into   natural     phenomena.       He    has 


THE  OLDER  EMBRYOLOGY 


unconsciously  given  the  best  description  of  himself  in  his 
famous  saying:  "  No  created  mind  can  penetrate  into  the  heart 
of  Nature ;  happy  the  man  to  whom  she  does  but  show  the  outer 
shell."  Goethe  made  the  best  reply  to  this  "  shell  theory  " 
of  observation  in  the  noble  poem  which  closes  with  the  words: 
"'Nature  has  neither  kernel  nor  shell;  she  is  all  one.  Try 
yourself  whether  you  are  either  kernel  or  shell."  Yet  there 
has  been  no  lack,  even  of  late  years,  of  attempts  to  defend 
Haller's  "shell  theory."  Wilhelm  His,  especially,  has  made 
a  strange  effort  to  justify  it. 

Haller  made  a  vigorous  defence  of  the  "evolution  theory" 
in  his  famous  work,  Elementa  physiologiae,  affirming:  "There 
is  no  such  thing  as  formation  (nulla  est  epigenesis ).  No 
part  of  the  animal  frame  is  made  before  another;  all  were 
made  together."  He  thus  denied  that  there  was  anv 
evolution  in  the  proper  sense  of  the  word,  and  even  went 
so  far  as  to  say  that  the  beard  existed  in  the  new-born 
child  and  the  antlers  in  the  hornless  fawn;  all  the  parts  were 
there  in  advance,  and  were  merely  hidden  from  the  eye  of  man 
for  the  time  being.  Haller  even  calculated  the  number  of 
human  beings  that  God  must  have  created  on  the  sixth  day 
and  stored  away  in  Eve's  ovary.  He  put  the  number  at 
200,000  millions,  assuming  the  age  of  the  world  to  be  6,000 
years,  the  average  age  of  a  human  being  to  be  thirty  years, 
and  the  population  of  the  world  at  that  time  to  be  1,000 
millions.  And  the  famous  Haller  maintained  all  this  non- 
sense, in  spite  of  its  ridiculous  consequences,  even  after  Wolff 
had  discovered  the  real  course  of  embryonic  development  and 
established  it  by  direct  observation! 

Among  the  philosophers  of  the  time  the  distinguished 
Leibnitz  was  the  chief  defender  of  the  "  preformation  theorv," 
and  by  his  authority  and  literary  prestige  won  many  adherents 
to  it.  Supported  by  his  system  of  monads,  according  to 
which  body  and  soul  are  united  in  inseparable  association  and 
by  their  union  form  the  individual,  or  the  "monad,"  Leibnitz 
consistently  extended  the  "  scatulation  theory "  to  the  soul, 
and  held  that  this  was  no  more  evolved  than  the  body.  He 
says,  for  instance,  in  his  Theodicec:  "  I  mean  that  these  souls, 


THE  OLDER  EMBRYOLOGY 


which  one  day  are  to  be  the  souls  of  men,  are  present  in  the 
.seed,  like  those  of  other  species;  in  such  wise  that  they  existed 
in  our  ancestors  as  far  back  as  Adam,  or  from  the  beginning 
of  the  world,  in  the  forms  of  organised  bodies." 

The  theory  seemed  to  receive  considerable  support  from 
the  observations  of  one  of  its  most  zealous  supporters, 
Bonnet.  In  1745  lie  discovered,  in  the  plant-louse,  a  case  of 
parthenogenesis,  or  virgin-birth,  an  interesting  form  of 
reproduction  that  has  lately  been  found  by  Siebold  and 
others  among  various  classes  of  the  articulata,  especial ly 
crabs  and  insects.  Among  these  and  other  animals  of  certain 
lower  species  the  female  may  reproduce  for  several  generations 
without  having  been  fertilised  by  the  male.  These  ova  that 
do  not  need  fertilisation  are  called  "  false  ova,"  pseudova  or 
spores.  Bonnet  saw  that  a  female  plant-louse,  which  he  had 
kept  in  cloistral  isolation,  and  rigidly  removed  from  contact 
with  males,  had  on  the  eleventh  day  (after  forming  a  new  skin 
for  the  fourth  time)  a  living  daughter,  and  during  the  next 
twenty  days  ninety-four  other  daughters  ;  and  that  all  of  them 
went  on  to  reproduce  in  the  same  way  without  any  contact 
with  males.  It  seemed  as  if  this  furnished  an  irrefutable 
proof  o(  the  truth  of  the  scatulation  theory,  as  it  was  held  by 
the  OvulistS  ;  it  is  not  surprising  to  find  that  the  theory  then 
secured  general  acceptance. 

This  was  the  condition  of  things  when  suddenlv,  in  1759, 
Caspar  Friedrich  Wolff  appeared,  and  dealt  a  fatal  blow  at 
the  whole  preformation  theory  with  his  new  theory  of 
epigenesis.  Wolff,  the  son  of  a  Berlin  tailor,  was  born  in 
1733,  and  went  through  his  scientific  and  medical  studies, 
first  at  Berlin  under  the  famous  anatomist  Meckel,  and  after- 
wards at  Halle.  Here  he  secured  his  doctorate  in  his  twenty- 
sixth  year,  and  in  his  academic  dissertation  (November  2<Sth, 
1759)  expounded  the  new  theory  of  a  real  development,  the 
theoria  generationis,  on  a  basis  of  epigenesis.  This  treatise 
is,  in  spite  of  its  smallness  and  its  obscure  phraseology,  one 
of  the  most  valuable  in  the  whole  range  of  biological  litera- 
ture. It  is  equally  distinguished  for  the  mass  of  new  and 
careful    observations    it    contains,   and    the    far-reaching   and 


THE  OLDER  EMBRYOLOGY 


pregnant  ideas  which  the  author  everywhere  extracts  from  his 
observations  and  builds  into  a  luminous  and  accurate  theory 
of  generation.  Nevertheless,  it  met  with  no  success  at  the 
time.  Although  scientific  studies  were  then  assiduously 
cultivated  owing  to  the  impulse  given  by  Linne — although 
botanists  and  zoologists  were  no  longer  counted  by  dozens, 
but  by  hundreds,  hardly  any  notice  was  taken  of  Wolff's 
theory.  Even  when  he  established  the  truth  of  epigenesis 
by  the  most  rigorous  observations,  and  demolished  the  airy 
structure  of  the  preformation  theory,  the  "  exact  "  scientist 
Haller  proved  one  of  the  most  strenuous  supporters  of  the 
old  theory,  and  rejected  Wolft's  correct  view  with  a  dic- 
tatorial Nulla  est  epigenesis.  He  even  went  on  to  say 
that  religion  was  menaced  by  the  new  theory  !  It  is  not 
surprising  that  the  whole  of  the  physiologists  of  the  second 
half  of  the  eighteenth  century  submitted  to  the  ruling  of  this 
physiological  pontiff,  and  attacked  the  theory  of  epigenesis  as 
a  dangerous  innovation.  It  was  not  until  more  than  fifty 
years  afterwards  that  Wolffs  work  was  appreciated.  Only 
when  Meckel  translated  into  German  in  1812  another  valuable 
work  of  Wolff's  on  The  Formation  of  the  Alimentary  Canal 
(written  in  1768),  and  called  attention  to  its  great  importance, 
did  people  begin  to  think  of  him  once  more  ;  yet  this  obscure 
writer  had  evinced  a  profounder  insight  into  the  nature  of  the 
living  organism  than  any  other  scientist  of  the  eighteenth 
century. 

Thus,  as  has  so  often  happened  in  the  history  of  thought, 
the  newly-discovered  truth  was  crushed  by  the  powerful 
untruth,  supported  by  the  might  of  authority.  The 
luminous  theory  of  epigenesis  could  not  penetrate  the  mists 
of  the  preformation  theory,  and  its  gifted  author  succumbed 
to  his  enemies  in  the  fight  for  truth.  All  further  advance  in 
embryology  was  thus  prevented  for  the  time  being.  It  was 
the  more  unfortunate  as  Wolff  was  compelled  by  the  poverty 
of  his  circumstances  to  leave  Germany  on  account  of  this 
opposition.  Henceforward  without  resources,  he  could  only 
complete  his  classical  work  under  the  most  pressing  diffi- 
culties, and  had  then  to  earn  his  living  by  medical  practice. 


LIBRAHT 

THE  OLDER  KM/lKYOLOGm       tylT)   \J         55 


During  the  Seven  Years'  War  he  worked  in  the  hospitals  of 
Schleswig,  and  gave  brilliant  lectures  on  anatomy  in  the 
field-hospital  at  Breslau,  and  so  attracted  the  attention  of  the 
director-general  of  hospitals,  Cothenius.  At  the  conclusion 
of  the  war  this  patron  endeavoured  to  obtain  a  professorship 
for  Wolff  at  Berlin.  But  he  failed,  owing  to  the  opposition 
of  the  narrow-minded  professors  of  the  Berlin  Medico- 
chirurgical  College,  who  were  ill-disposed  to  scientific 
progress.  They  declared  the  epigenesis  theory  to  be  a 
deadly  heresy,  just  as  they  condemned  the  theory  of  descent 
only  a  few  decades  ago.  Although  Cothenius  and  other 
admirers  struggled  bravely  for  Wolff,  they  could  not  even 
get  him  permission  to  give  public  lectures  on  physiology  at 
Berlin.  In  the  end  Wolff  was  compelled  to  accept  an 
honourable  position  that  was  offered  to  him  in  1766  by 
Catharine  of  Russia.  He  went  to  St.  Petersburg,  and 
continued  his  researches  there  for  twenty-seven  years. 

Wolff's  ideas  led  to  an  appreciable  advance  over  the  whole 
field  of  biology.  There  is  such  a  vast  number  of  new  and 
important  observations  and  pregnant  thoughts  in  his  writings 
that  we  have  only  gradually  learned  to  appreciate  them 
rightly  in  the  course  of  the  nineteenth  century.  He  opened 
up  the  true  path  for  research  in  many  directions.  In  the  first 
place,  his  theory  of  epigenesis  gave  us  our  first  real  insight 
into  the  nature  of  embryonic  development.  He  showed  con- 
vincingly that  the  development  of  every  organism  consists  of 
a  series  of  new  formations,  and  that  there  is  no  trace  whatever 
of  the  complete  form  either  in  the  ovum  or  the  spermatozoon. 
On  the  contrary,  these  are  quite  simple  bodies,  with  a  very 
different  purport.  The  embryo  which  is  developed  from 
them  is  also  quite  different,  in  its  internal  arrangement  and 
outer  configuration,  from  the  complete  organism.  There  is 
no  trace  whatever  of  preformation  or  in-folding  of  organs. 
To-day  we  can  scarcely  call  epigenesis  a  theory,  because  we 
are  convinced  it  is  a  fact,  and  can  demonstrate  it  at  any 
moment  with  the  aid  of  the  microscope. 

Wolff  furnished  the  conclusive  empirical  proof  of  his 
theory  in    his   classic   dissertation  on   The  Formation  of  the 


THE  OLDER  EMBRYOLOGY 


Alimentary  Canal  (i 768).  In  its  complete  state  the  alimen- 
tary canal  of  the  hen  is  a  long  and  complex  tube,  with  which 
the  lungs,  liver,  salivary  glands,  and  many  other  small 
glands,  are  connected.  Wolff  showed  that  in  the  early 
stages  of  the  embryonic  chick  there  is  no  trace  whatever  of 
this  complicated  tube  with  all  its  dependencies,  but  instead  of 
it  only  a  flat,  leaf-shaped  body  ;  that,  in  fact,  the  whole 
embryo  has  at  first  the  appearance  of  a  flat,  oval-shaped  leaf. 
When  we  remember  how  difficult  the  exact  observation  of  so 
fine  and  delicate  a  structure  as  the  early  leaf-shaped  body  of 
the  chick  must  have  been  with  the  poor  microscopes  then  in 
use,  we  must  admire  the  rare  faculty  for  observation  which 
enabled  Wolff  to  make  the  most  important  discoveries  in  this 
most  difficult  part  of  embryology.  By  this  laborious  research 
he  reached  the  correct  opinion  that  the  embryonic  body  of  all 
the  higher  animals,  such  as  the  birds,  is  for  some  time  merely 
a  flat,  thin,  leaf-shaped  disk — consisting  at  first  of  one,  but 
afterwards  of  several,  layers.  The  lowest  of  these  layers  is 
the  alimentary  canal,  and  Wolff  followed  its  development 
from  its  commencement  to  its  completion.  He  showed  how 
this  leaf-shaped  structure  first  turns  into  a  groove,  then  the 
margins  of  this  groove  fold  together  and  form  a  closed  canal, 
and  at  length  the  two  external  openings  of  the  tube  (the  mouth 
and  anus)  appear. 

Moreover,  the  important  fact  that  the  other  systems  of 
organs  are  developed  in  the  same  way,  from  tubes  formed  out 
of  simple  layers,  did  not  escape  Wolff.  The  nervous  system, 
muscular  system,  and  vascular  (blood-vessel)  system,  with  all 
the  organs  appertaining  thereto,  are,  like  the  alimentary 
system,  developed  out  of  simple  leaf-shaped  structures. 
Hence,  Wolff  came  to  the  view  by  1768  which  Pander 
developed  in  the  Theory  of  Germinal  Layers  fifty  years 
afterwards.  The  words  in  which  Wolff  anticipates  the  chief 
feature  of  this  are  so  remarkable  that  they  deserve  to  be 
quoted  in  full  : — 

This  wonderful  analogy  between  parts  that  seem  to  be  so  widely  removed 
from  each  other  in  Nature — no  product  of  the  imagination,  but  supported  by 
the  most  confident  observations — merits  the  attention  of  physiologists  in  the 
highest  degree,  for  it  must  be  admitted  to  have  a  profound  significance,  and  to 


THE  OLDER  EMBRYOLOGY  35 


in'  intimately  connected  with  the  generation  and  the  nature  of  animals.  It 
seems  as  if,  at  various  and  successive  stages,  different  systems  are  formed 
after  the  same  type,  and  those  then  unite  to  form  the  complete  animal ;  and  as 
if  these  really  resemble  each  other  in  spite  of  their  differences.  Tin-  first 
system  to  be  produced  and  take  definite  shape  is  the  nervous  system.  When 
this  is  done,  the  mass  of  muscle  which  constitutes  the  embryo  takes  shape  after 
the  same  fashion.  To  this  succeeds  a  third  system,  that  of  the  blood-vessels, 
which  is  not  so  unlike  the  first  as  to  prevent  us  from  seeing  in  it  the  form  which 
is  common  to  all  three.  After  this  comes  the  fourth,  the  alimentary  canal, 
which  again  is  constructed  on  the  same  type,  and  resembles  the  other  three,  in 
being  a  complete  and  self-contained  whole. 

In  this  important  discovery  Wolff  laid  the  foundation  of 
the  theory  of  germinal  layers,  which  was  not  fully  developed 
until  much  later  by  Pander  (1817)  and  Baer  (1828).  Wolff's 
principles  are  not  literally  correct;  but  he  comes  as  near  to 
the  truth  in  them  as  was  possible  at  that  time,  and  could  be 
expected  of  him. 

Wolff  owes  a  great  deal  of  his  success  in  forming  his 
comprehensive  theory  to  the  fact  that  he  was  as  distinguished 
in  botany  as  in  zoology.  He  studied  at  the  same  time  the 
development  of  plants,  and  was  the  first  to  establish  in 
botany  the  theory  which  Goethe  afterwards  developed  in  his 
famous  work  on  the  metamorphosis  of  plants.  Wolff  had 
already  shown  that  all  the  different  parts  of  the  plant  could 
be  reduced  to  the  leaf  as  the  fundamental  type.  The  flower 
and  the  fruit,  with  all  their  parts,  are  merely  modified  leaves. 
The  knowledge  of  this  must  have  much  surprised  Wolff,  as 
he  had  found  a  simple  leaf-shaped  structure  to  be  the  first 
form  of  the  embryonic  body  of  the  animal  as  well. 

Thus  we  find  in  Wolff  the  germs  of  the  two  theories  which 
other  and  much  later  scientists  were  to  make  the  basis  of  a 
morphological  comprehension  of  the  plant  and  the  animal. 
But  our  admiration  of  this  gifted  genius  increases  when  we 
find  that  he  was  also  the  precursor  of  the  famous  cellular 
theory.  Wolff  had,  as  Huxley  showed,  a  clear  presentiment 
of  this  cardinal  theory,  since  he  recognised  small  microscopic 
globules  as  the  elementary  parts  out  of  which  the  germinal 
layers  arose. 

Finally,  I  must  invite  special  attention  to  the  mechanical 
character  of  the  profound  philosophic  reflections  which  Wolff 
always  added  to  his   remarkable  observations.      He   was   a 


36  THE  OLDER  EMBRYOLOGY 

great  monistic  philosopher,  in  the  best  meaning  of  the  word. 
It  is  unfortunate  that  his  philosophic  discoveries  were  ignored 
as  completely  as  his  observations  for  more  than  half  a  century. 
We  must  be  all  the  more  careful  to  emphasise  the  fact  of  their 
clear  monistic  tendency. 


CHAPTER  III. 
MODERN    HMBRYOLOGY 

Karl  Ernst  von  Baer  as  the  chief  successor  to  Wolff  The  Wiirtzburg  school 
of  embryologists  :  Dollinger,  Pander,  Baer.  The  disk-shaped  germ  divides 
first  into  two  germinal  layers,  and  these  in  turn  sub-divide  into  two  each. 
Their  transformation  into  tubes.  Baer's  discovery  of  the  human  ovum,  the 
germinal  vesicle,  and  the  axial  rod.  The  lour  types  of  development  in  the 
lour  chief  animal  groups.  Baer's  law  of  the  type  of  development  and  the 
Stage  o(  construction.  Explanation  of  this  law  by  the  theory  of  selection. 
Baer's  successors — Ralhke,  Johannes  Mtiller,  Bischoff,  Kolliker.  The 
cellular  theory — Schleiden,  Schwann.  Its  application  to  embryology— 
Kemak.  Reaction  in  embryology  ;  Reichert  and  His.  The  mechanical 
theories  of  His  ;  the  "  tailor  theory  "  and  the  "  theory  of  parablasts. "  Chief 
embryo  and  secondary  embryo.  Symbiosis  of  the  vertebrates.  Mechanical 
explanation  of  the  embryonic  processes.  The gastraea-theory.  Homology- 
of  the  two  primary  layers.  Protozoa  and  metazoa.  Ccelenterata  and 
coelomaria.  The  coslum-theory  o(  Hertwig.  The  four  secondary  embry- 
onic layers.  Progress  in  recent  embryology.  Experimental  embryology. 
Mechanical  embryology. 

We  may  distinguish  three  chief  periods  in  the  growth  of  our 
science  of  human  embryology.  The  first  has  been  considered 
in  the  preceding  chapter;  it  embraces  the  whole  of  the  prepa- 
ratory period  of  research,  and  extends  from  Aristotle  to 
Caspar  Friedrich  Wolff,  or  to  the  year  1759,  in  which  the 
epoch-making  Thcoria  generationis  was  published.  The 
second  period,  with  which  we  have  now  to  deal,  lasts  about 
a  century — that  is  to  say,  until  the  appearance  of  Darwin's 
Origin  of  Species,  which  brought  about  a  change  in  the  very 
foundations  of  biology,  and,  in  particular,  of  embryology. 
The  third  period  begins  with  Darwin.  When  we  say  that 
the  second  period  lasted  a  full  century,  we  must  remember 
that  Wolff's  work  had  remained  almost  unnoticed  during  half 
the  time — namely,  until  the  year  1812.  During  the  whole  of 
these  fifty-three  years  not  a  single  book  that  appeared  followed 
up  the  path  that  Wolff  had  opened,  or  extended  his  theory  o\ 
embryonic  development.  We  merely  find  his  views — per- 
fectly correct  views,  based  on  extensive  observations  of  fact— 
37 


38  MODERN  EMBRYOLOGY 


mentioned  here  and  there  as  erroneous;  their  opponents,  who 
adhered  to  the  dominant  theory  of  preformation,  did  not 
even  deign  to  reply  to  them.  This  unjust  treatment  was 
chiefly  due  to  the  extraordinary  authority  of  Albrecht  von 
Haller;  it  is  one  of  the  most  astonishing  instances  of  a  great 
authority,  as  such,  preventing  for  a  long  time  the  recognition 
of  established  facts. 

The  general  ignorance  of  Wolff's  work  was  so  great  that 
at  the  beginning  of  the  nineteenth  century  two  scientists  of 
Jena,  Oken  (1806)  and  Kieser  (1810),  began  independent 
research  into  the  development  of  the  alimentary  canal  of  the 
chick,  and  hit  upon  the  right  clue  to  the  embryonic  puzzle, 
without  knowing  a  word  about  Wolff's  important  treatise  on 
the  same  subject.  They  were  treading  in  his  very  footsteps 
without  suspecting  it.  This  can  be  easily  proved  from  the 
fact  that  they  did  not  travel  as  far  as  Wolff.  It  was  not 
until  Meckel  translated  into  German  Wolff's  book  on  the 
alimentary  system,  and  pointed  out  its  great  importance,  that 
the  eyes  of  anatomists  and  physiologists  were  suddenly 
opened.  At  once  a  number  of  biologists  instituted  fresh 
embryological  inquiries,  and  began  to  confirm  Wolff's  theory 
of  epigenesis. 

This  resuscitation  of  embryology  and  development  of  the 
epigenesis-theory  was  chiefly  connected  with  the  university  ot 
Wiirtzburg.  One  of  the  professors  there  at  that  time  was 
Dollinger,  an  eminent  biologist,  and  father  of  the  famous 
Catholic  historian  who  later  distinguished  himself  by  his 
opposition  to  the  new  dogma  of  papal  infallibility.  Dollinger 
was  both  a  profound  thinker  and  an  accurate  observer.  He 
took  the  keenest  interest  in  embryology,  and  worked  at  it  a 
good  deal.  However,  he  is  not  himself  responsible  for  any 
important  result  in  this  field.  In  1816  a  young  medical 
doctor,  whom  we  may  at  once  designate  as  Wolff's  chief 
successor,  Karl  Ernst  von  Baer,  came  to  Wiirtzburg.  Baer's 
conversations  with  Dollinger  on  embryology  led  to  a  fresh 
series  of  most  extensive  investigations.  Dollinger  had 
expressed  a  wish  that  some  young  scientist  should  begin 
again  under  his  guidance  an   independent  inquiry  into  the 


MOPKhW  KMRRYOLOG Y 


development  of  the  chick  during  the  hatching  of  the  egg. 
As  neither  he  nor  Baer  had  money  enough  to  pay  for  an 
incubator  and  the  proper  control  of  the  experiments,  and  for  a 
competent  artist  to  illustrate  the  various  stages  observed,  the 
lead  of  the  enterprise  was  given  to  Christian  Pander,  a 
wealthy  friend  of  Baer's,  who  had  been  induced  by  Baer  to 
come  to  Wiirtzburg.  An  able  engraver,  Dalton,  was 
engaged  to  do  the  copper-plates. 

Thus  was  formed,  in  the  words  of  Baer,  "an  association 
of  memorable  importance  to  science,  in  which  a  veteran  of 
physiological  research  (Dollinger),  an  ardent  scientific 
neophyte  (Pander),  and  an  unrivalled  artist  (Dalton),  joined 
forces  in  order  to  provide  a  firm  foundation  for  the 
embryology  of  the  animal  organism."  In  a  short  time  the 
embryology  of  the  chick,  in  which  Baer  was  taking  the 
greatest  indirect  interest,  was  so  far  advanced  that  Pander 
was  able  to  sketch  the  main  features  of  it  on  the  ground  of 
Wolffs  theory  in  the  dissertation  he  published  in  1817.  He 
clearly  enunciated  the  theory  of  germinal  layers  which 
Wolff  had  anticipated,  and  established  the  truth  of  Wolff's 
idea  of  a  development  of  the  complicated  systems  of  organs 
out  of  simple  leaf-shaped  primitive  structures.  According  to 
Pander,  the  leaf-shaped  object  in  the  hen's  egg  divides, 
before  the  incubation  has  proceeded  twelve  hours,  into  two 
different  layers,  an  external  serous  layer  and  an  internal 
mucous  layer;  between  the  two  there  developes  later  a  third 
layer,  the  vascular  (blood-vessel)  layer.1 

Karl  Ernst  von  Baer,  who  had  set  afoot  Pander's  investi- 
gation, and  had  shown  the  liveliest  interest  in  it  after  Pander's 
departure  from  Wiirtzburg,  began  his  own  much  more  com- 
prehensive research  in  1819.  He  published  the  mature  result 
nine  years  afterwards  in  his  famous  work,  Animal  Embryo- 
logy :  Observation  and  Reflection  (not  translated).  This 
classic  work  still  remains  a  model  of  careful  observation 
united  to  profound  philosophic  speculation.  The  first  part 
appeared   in    1828,  the  second  in    1837.     The  book  proved  to 

1  I  need  scarcely  note  thai  the  technical  terms  which  are  bound  to  creep 
into  this  chapter  will  be  fully  understood  later  on.— Trans. 


MODERN  EMBRYOLOGY 


be  the  foundation  on  which  the  whole  science  of  embryology 
has  built  down  to  our  own  day.  It  so  far  surpassed  its 
predecessors,  and  Pander  in  particular,  that  it  has  become, 
after  Wolff's  work,  the  chief  base  of  modern  embryology.  As 
Baer  was  one  of  the  greatest  scientists  of  the  nineteenth 
century,  and  exercised  considerable  influence  on  other 
branches  of  biology  as  well,  it  will  be  interesting  to  add  a 
few  points  with  regard  to  his  life. 

Karl  Ernst  von  Baer  was  born  at  Esthland,  in  Piep,  a 
small  estate  belonging  to  his  father,  in  1794.  He  studied 
from  1810  to  1814  at  Dorpat,  and  went  from  there  to  Wiirtz- 
burg,  where  Dollinger  not  only  initiated  him  to  comparative 
anatomy  and  embryology,  but  had  a  very  beneficial  general 
influence  over  him  in  the  way  of  scientific  method.  From 
Wiirtzburg  he  went  to  Berlin,  and  then,  at  the  invitation  of 
the  physiologist  Burdach,  to  Konigsberg,  where,  with  few 
interruptions,  he  lectured  on  zoology  and  embryology  until 
1834,  ar,d  wrote  his  chief  works.  .  In  1834  he  went  to  St. 
Petersburg  and  became  a  member  of  the  academy  of  that 
city.  Here  he  almost  deserted  his  earlier  field,  and  engaged 
in  various  kinds  of  research  of  a  quite  different  character, 
especially  in  geography,  geology,  ethnography,  and  anthro- 
pology. During  the  last  forty  years  his  general  views 
gradually  altered,  as  I  have  described  in  my  Riddle  of  the 
Universe.  In  earlier  years  he  had  been  a  consistent  supporter 
of  the  monistic  system.  He  had  in  his  chief  work  (especially 
in  the  preface  and  at  the  close)  insisted  on  the  unity  and 
naturalness  of  evolution.  But  in  later  years  he  leaned  more 
and  more  to  mystical  and  teleological  considerations;  and,  in 
the  end,  his  anthropistic  dualism  led  him  to  embrace  a 
curious  form  of  theology.  He  spent  his  last  years  at  Dorpat, 
where  he  died  in  1876.  His  most  important  works  are 
certainly  those  dealing  with  animal  embryology,  and  were  all 
written  in  Konigsberg,  though  partly  published  elsewhere. 
Their  great  service  extends,  like  that  of  Baer,  over  the 
whole  field  of  embryology  in  many  different  directions. 

Baer  built  up  the  theory  of  germinal  layers,  as  a  whole 
and    in   detail,  so  clearly  and  solidly  that    it   has   been    the 


MODERN  EMliRYOLOGY 


Starting-point  of  ontogenetic  research  ever  since.  He  taught 
that  in  all  the  vertehrates  first  two  and  then  four  of  these 
germinal  layers  are  formed;  and  that  the  earliest  rudimentary 
organs  o(  the  body  arise  by  the  conversion  of  these  layers 
into  tubes.  He  described  the  first  appearance  of  the  verte- 
brate embryo,  as  it  may  be  seen  in  the  globular  yelk  of  the 
fertilised  egg,  as  an  oval  disk  which  first  divides  into  two 
layers.  From  the  upper  or  animal  layer  are  developed  all 
the  organs  which  accomplish  the  phenomena  of  animal  life — 
the  functions  of  sensation  and  motion,  and  the  covering  of 
the  body.  From  the  lower  or  vegetative  layer  come  the 
organs  which  effect  the  vegetative  life  of  the  organism — 
nutrition,  digestion,  blood-formation,  respiration,  secretion, 
reproduction,  etc. 

Each  of  these  original  layers  divides,  according  to  Baer, 
into  two  thinner  and  superimposed  layers  or  plates.  He  calls 
the  two  plates  of  the  animal  layer,  the  skin-stratum  and 
muscle-stratum.  From  the  upper  of  these  plates,  the  skin- 
stratum,  the  external  skin,  or  outer  covering  of  the  body,  the 
central  nervous  system,  and  the  sense-organs,  are  formed. 
From  the  lower,  or  muscle-stratum,  the  muscles,  or  fleshy 
parts  and  the  bony  skeleton — in  a  word,  the  motor  organs — 
are  evolved.  In  the  same  way,  Baer  said,  the  lower  or 
vegetative  layer  splits  into  two  plates,  which  he  calls  the 
vascular-stratum  and  the  mucous-stratum.  From  the  outer 
of  the  two  (the  vascular)  the  heart,  blood-vessels,  spleen,  and 
the  other  vascular  glands,  the  kidneys,  and  sexual  glands, 
are  formed.  From  the  fourth  or  mucous  layer,  in  fine,  we  get 
the  internal  and  digestive  lining  of  the  alimentary  canal  and 
all  its  dependencies,  the  liver,  lungs,  salivary  glands,  etc. 
Baer  had,  in  the  main,  correctly  judged  the  significance  of 
these  four  secondary  embryonic  lavers,  and  he  followed  the 
conversion  of  them  into  the  tube-shaped  primitive  organs 
with  great  perspicacity.  He  first  solved  the  difficult  problem 
of  the  transformation  of  this  four-fold,  flat,  leaf-shaped,  em- 
bryonic disk  into  the  complete  vertebrate  body,  through  the 
conversion  of  the  layers  or  plates  into  tubes.  The  flat  leaves 
bend  themselves  in  obedience  to  certain  laws  of  growth;  the 


MODERN  EMBRYOLOGY 


borders  of  the  curling  plates  approach  nearer  and  nearer; 
until  at  last  they  come  into  actual  contact.  Thus  out  of  the 
flat  gut-plate  is  formed  a  hollow  gut-tube,  out  of  the  flat  spinal 
plate  a  hollow  nerve-tube,  from  the  skin-plate  a  skin-tube,  and 
so  on. 

Among  the  many  great  services  which  Baer  rendered  to 
embryology,  especially  vertebrate  embryology,  we  must  not 
forget  his  discovery  of  the  human  ovum.  Earlier  scientists 
had,  as  a  rule,  of  course,  assumed  that  man  developed  out  of 
an  egg,  like  the  other  animals.  In  fact,  the  preformation 
theory  held  that  the  germs  of  the  whole  of  humanity  were 
stored  already  in  Eve's  ova.  But  the  real  ovum  escaped 
detection  until  the  year  1827.  This  ovum  is  extremely  small, 
being  a  tiny  round  vesicle  about  the  rht  of  an  inch  in  diameter; 
it  can  be  seen  under  very  favourable  circumstances  with  the 
naked  eye  as  a  tiny  particle,  but  is  otherwise  quite  invisible. 
This  particle  is  formed  in  the  ovary  inside  a  much  larger 
globule,  which  takes  the  name  of  the  Graafian  follicle,  from 
its  discoverer,  Graaf,  and  had  previously  been  regarded  as 
the  true  ovum.  However,  in  1827  Baer  proved  that  it  was 
not  the  real  ovum,  which  is  much  smaller,  and  is  contained 
within  the  follicle.  (Compare  the  end  of  the  twenty-ninth 
chapter.) 

Baer  was  also  the  first  to  observe  what  is  known  as  the 
segmentation  sphere  of  the  vertebrate  ;  that  is  to  say,  the 
globular  vesicle  which  first  developes  out  of  the  impregnated 
ovum,  and  the  thin  wall  of  which  is  made  up  of  a  single  layer 
of  regular,  polygonal  (many-cornered)  cells  (see  the  illustra- 
tion in  the  twelfth  chapter).  Another  discovery  of  his  that 
was  of  great  importance  in  constructing  the  vertebrate  stem 
and  the  characteristic  organisation  of  this  extensive  group  (to 
which  man  belongs)  was  the  detection  of  the  axial  rod,  or  the 
chorda  dorsalis.  This  is  a  long,  round,  cylindrical  rod  of 
cartilage  which  runs  down  the  longer  axis  of  the  vertebrate 
embryo  ;  it  appears  at  an  early  stage,  and  is  the  first  sketch 
of  the  spinal  column,  the  solid  skeletal  axis  of  the  vertebrate. 
In  the  lowest  of  the  vertebrates,  the  amphioxus,  the  internal 
skeleton  consists  only  of  this  cord  throughout  life.     But  even 


MlWKh'.X  EMBR  YOLOGY 


in  the  case  of  man  and  all  the  higher  vertebrates  it  is  round 
this  cord  that  the  spinal  column  and  the  brain  are  afterwards 
formed. 

However,  important  as  these  and  many  other  discoveries 
of  Baer's  were  in  vertebrate  embryology,  his  researches  were 
even  more  influential,  from  the  circumstance  that  he  was  the 
first  to  employ  the  comparative  method  in  studying  the 
development  of  the  animal  frame.  Baer  occupied  himself 
chiefly  with  the  embryology  of  vertebrates  (especially  the 
birds  and  fishes).  But  he  by  no  means  confined  his  attention 
to  these,  gradually  taking  the  various  groups  of  the  inverte- 
brates into  his  sphere  of  study.  As  the  general  result  of  his 
comparative  embrvological  research,  Baer  distinguished  four 
different  modes  of  development  and  four  corresponding 
groups  in  the  animal  world.  These  chief  groups  or  types 
are:  i,  the  vertebrata  ;  2,  the  articulata  ;  3,  the  mollusca  ; 
and  4,  all  the  lower  groups  which  were  then  wrongly 
comprehended  under  the  general  name  of  the  radiata. 
Georges  Cuvier  had  been  the  first  to  formulate  this  dis- 
tinction, in  1812.  He  showed  that  these  groups  present 
specific  differences  in  their  whole  internal  structure,  and  the 
connection  and  disposal  of  their  systems  of  organs  ;  and  that, 
on  the  other  hand,  all  the  animals  of  the  same  type — say,  the 
vertebrates — essentially  agreed  in  their  inner  structure  in  spite 
of  the  greatest  superficial  differences.  But  Baer  proved  that 
these  four  groups  are  also  quite  differently  developed  from 
the  ovum  ;  and  that  the  series  of  embryonic  forms  is  the 
same  throughout  for  animals  of  the  same  type,  but  different 
in  the  case  of  other  animals.  Up  to  that  time  the  chief  aim 
in  the  classification  of  the  animal  kingdom  was  to  arrange 
all  the  animals  from  lowest  to  highest,  from  the  infusorium  to 
man,  in  one  long  and  continuous  series.  The  erroneous  idea 
prevailed  nearly  everywhere  that  there  was  one  uninterrupted 
chain  of  evolution  from  the  lowest  animal  to  the  highest. 
Cuvier  and  Baer  proved  that  this  view  was  false,  and  that  we 
must  distinguish  four  totally  different  types  of  animals,  on 
the  ground  of  anatomic  structure  and  embryonic  development. 

Following   up  this  discovery,  Baer  came  to  formulate  a 


MODERN  EMBRYOLOGY 


very  important  law,  which  is  called  after  him  Baer's  law,  and 
which  he  himself  expressed  in  these  words  : — 

The  development  of  an  individual  of  any  animal  type  is  characterised  by 
two  features :  firstly,  by  the  progressive  construction  of  the  animal  body 
through  a  continuous  histological  and  morphological  segmentation  ;  secondly, 
by  an  advance  from  a  more  general  to  a  more  special  form  of  structure.  The 
degree  of  development  of  the  organism  consists  in  the  greater  or  less  measure 
of  the  heterogeneity  of  its  elementary  parts  and  of  the  several  sections  of  its 
connected  system  ;  in  other  words,  in  its  greater  histological  and  morpholo- 
gical subdivision  (or  differentiation).  On  the  other  hand,  the  type  consists  in 
the  disposition  of  the  organic  elements  in  the  organs.  The  type  is  an  entirely 
different  thing  from  the  degree  of  development  ;  the  same  type  may  be  found 
in  various  stages  of  development,  and,  vice  versfi,  the  same  stage  of  develop- 
ment may  be  had  in  different  types. 

Hence  it  is  that  the  most  advanced  animals  of  each  type — 
for  instance,  the  highest  articulata  and  mollusca — are  much 
more  highly  organised  (or  more  effectively  differentiated) 
than  the  lowest  animals  of  every  other  type,  such  as  the 
lowest  vertebrates  and  the  echinoderms. 

This  law  of  Baer  has  proved  of  great  service  in  our  study 
of  animal  organisation,  although  we  were  not  in  a  position  to 
understand  and  appreciate  its  real  significance  until  Darwin 
appeared.  I  may  add  that  a  thorough  comprehension  of  it  is 
only  possible  in  the  light  of  the  theory  of  descent,  and  after 
recognising  the  important  part  that  heredity  and  adaptation 
play  in  the  production  of  organic  forms.  As  I  showed  in  my 
Generelle  Morphologie  (Band  II.,  §  10),  the  type  of  develop- 
ment is  a  mechanical  result  of  heredity  ;  but  the  degree  of 
development  is  a  mechanical  consequence  of  adaptation. 
Heredity  and  adaptation  are  the  mechanical  agents  in 
organic  construction  which  Darwin's  theory  of  selection 
introduced  into  embryology,  and  through  which  we  have  at 
last  come  to  understand  Baer's  law. 

Baer's  epoch-making  works  aroused  an  extraordinary  and 
widespread  interest  in  embryological  research.  Immediately 
afterwards  we  find  a  great  number  of  observers  at  work  in  the 
newly  opened  field,  enlarging  it  in  a  very  short  time  with 
great  energy  by  their  various  discoveries  in  detail.  Next  to 
Baer's  comes  the  admirable  work  of  Heinrich  Rathke,  of 
Konigsberg  (died  i860)  ;  he  made  an  extensive  study  of  the 
embryology,  not   only   of  the   invertebrates   (crabs,  insects, 


MODERN  EMBRYOLOGY  45 

molluscs),  but  also,  and  particularly,  of  the  vertebrates 
(fishes,  tortoises,  serpents,  crocodiles,  etc.).  We  owe  the 
first  comprehensive  studies  of  mammal  embryology  to  the 
careful  research  of  Wilhelm  Bischoff,  of  Munich  ;  his 
embryology  of  the  hare  (1S40),  the  dog  (1842),  the  guinea- 
pig  (1852),  and  the  doe  (1854),  st'"  f°rm  classical  studies. 
About  the  same  time  a  great  impetus  was  given  to  the 
embryology  of  the  invertebrates.  The  way  was  opened 
through  this  obscure  province  by  the  studies  of  the  famous 
Berlin  zoologist,  Johannes  Miiller,  on  the  echinoderma. 
I  le  was  followed  by  Albert  Kolliker,  of  Wiirtzburg,  writing 
on  the  cuttle-fish  (or  the  cephalopods),  Siebold  and  Huxley 
Oil  worms  and  zoophytes,  Fritz  Miiller  (Desterro)  on  the 
Crustacea,  Weismann  on  insects,  and  so  on.  The  number  of 
workers  in  this  field  has  greatly  increased  of  late,  and  a 
quantity  of  new  and  astonishing  discoveries  have  been  made. 
One  notices,  in  several  of  these  recent  works  on  embryology, 
that  their  authors  are  too  little  acquainted  with  comparative 
anatomy  and  classification.  Paleontology  is,  unfortunately, 
altogether  neglected  by  many  of  these  new  workers,  although 
this  interesting  science  furnishes  most  important  facts  for 
phylogeny,  and  thus  often  proves  of  very  great  service  in 
ontogeny. 

A  very  important  advance  was  made  in  our  science  in 
1839,  when  the  cellular  theory  was  established,  and  a  new 
field  of  inquiry  bearing  on  embryology  was  suddenly  opened. 
When  the  famous  botanist,  M.  Schleiden,  of  Jena,  showed  in 
1838,  with  the  aid  of  the  microscope,  that  every  plant  was 
made  up  of  innumerable  elementary  parts,  which  we  call 
cells,  a  pupil  of  Johannes  Miiller  at  Berlin,  Theodor 
Schwann,  applied  the  discovery  at  once  to  the  animal 
organism.  He  showed  that  in  the  animal  body  as  well, 
when  we  examine  its  tissues  in  the  microscope,  we  find  these 
cells  everywhere  to  be  the  elementary  units.  All  the 
different  tissues  of  the  organism,  especially  the  very  dissimilar 
tissues  of  the  nerves,  muscles,  bones,  external  skin,  mucous 
lining  etc.,  are  originally  formed  out  of  cells;  and  this  is  also 
true  of  all  the  tissues  of  the  plant.     These  cells  are  separate 


46  MODERN  EMBRYOLOGY 

living  beings;  they  are  the  citizens  of  the  State  which  the 
entire  multicellular  organism  seems  to  be.  This  important 
discovery  was  bound  to  be  of  service  to  embryology,  as  it 
raised  a  number  of  new  questions.  What  is  the  relation  of 
the  cells  to  the  germinal  layers?  Are  the  germinal  layers 
composed  of  cells,  and  what  is  their  relation  to  the  cells  of 
the  tissues  that  form  later?  How  does  the  ovum  stand  in  the 
cellular  theory  ?  Is  the  ovum  itself  a  cell,  or  is  it  composed 
of  cells?  These  important  questions  were  now  imposed  on 
the  embryologist  by  the  cellular  theory. 

The  most  notable  effort  to  answer  these  questions — which 
were  attacked  on  all  sides  by  different  students — is  contained 
in  the  famous  work,  Inquiries  into  the  Development  of  the 
Vertebrates  (not  translated)  of  Robert  Remak,  of  Berlin 
(1851).  This  gifted  scientist  succeeded  in  mastering,  by  a 
complete  reform  of  the  science,  the  great  difficulties  which 
the  cellular  theory  had  at  first  put  in  the  way  of  embryology. 
A  Berlin  anatomist,  Carl  Boguslaus  Reichert,  had  already 
attempted  to  explain  the  origin  of  the  tissues.  But  this 
attempt  was  bound  to  miscarry,  since  its  not  very  clear- 
headed author  lacked  a  sound  acquaintance  with  embryology 
and  the  cell  theory,  and  even  with  the  structure  and  develop- 
ment of  the  tissue  in  particular.  An  examination  of 
Reichert's  discoveries  shows  how  inaccurate  his  observations 
were,  and  how  false  the  conclusions  he  drew  from  them.  I 
need  only  give  one  illustration  :  he  believed  the  whole  of  the 
outer  germinal  layer,  from  which  the  most  important  organs 
are  developed  (the  brain,  spinal  cord,  skin,  etc.),  to  be  only  a 
temporary  integument  of  the  embryo,  which  had  nothing  to 
do  with  the  actual  construction  of  the  organism.  According 
to  him,  the  forms  of  the  various  organs  did  not  come  for  the 
most  part  from  the  original  germinal  layers,  but  arose 
independently  of  these  out  of  the  yelk,  and  were  only 
gradually  joined  to  the  layers.  Reichert's  perverse  studies 
of  embryology  only  obtained  a  certain  amount  of  passing 
attention  through  the  audacious  way  in  which  they  were 
pushed  and  the  attack  he  made  on  Baer's  theory  of  the 
germinal  layers  ;  and,  in  fact,  they  were  so  badly  presented 


MODERN  EMBRYOLOGY 


that  nobody  really  understood  them.  However,  on  that  very 
account  they  won  the  admiration  of  a  good  many  readers, 
who  felt  that  there  must  be  some  fund  of  wisdom  at  the  back 
of  all  these  cloudy  oracles  and  mysteries.  We  see  the  same 
thing  here  and  there  to-day,  especially  as  regards  the  confused 
writings  of  the  "mechanical  embryologists  "  (such  as  Dreisch 
and  his  colleagues). 

Remak  at  length  brought  order  into  the  dreadful  confusion 
that  Reichert  had  caused;  he  gave  a  perfectly  simple  explana- 
tion of  the  origin  of  the  tissues.  In  his  opinion  the  animal 
ovum  is  always  a  simple  cell :  the  germinal  layers  which 
develop  out  of  it  are  always  composed  of  cells  ;  and  these 
cells  that  constitute  the  germinal  layers  arise  simply  from  the 
continuous  and  repeated  cleaving  (segmentation)  of  the 
original  solitary  cell.  It  first  divides  into  two  and  then  into 
four  cells  ;  out  of  these  four  cells  are  born  eight,  then  sixteen, 
thirty-two,  and  so  on.  Thus,  in  the  embryonic  development 
of  everv  animal  and  plant  there  is  formed  first  of  all  out  of  the 
simple  egg  cell,  by  a  repeated  sub-division,  a  cluster  of  cells, 
as  Kolliker  had  already  stated  in  connection  with  the 
cephalopods  in  1844.  The  cells  of  this  group  spread  them- 
selves out  flat  and  form  leaves  or  plates  ;  each  of  these  leaves 
is  formed  exclusively  out  of  cells.  The  cells  of  different 
layers  assume  different  shapes,  increase,  and  differentiate ; 
and  in  the  end  there  is  a  further  cleavage  (differentiation)  and 
division  of  work  (ergonomy)  of  the  cells  within  the  layers, 
and  from  these  all  the  different  tissues  of  the  body  proceed. 

These  are  the  simple  foundations  of  histogeny,  or  the 
science  that  treats  of  the  development  of  the  tissues  (hista)y 
as  it  was  established  by  Remak  and  Kolliker.  Remak,  in 
determining  more  closely  the  part  which  the  different  germinal 
layers  play  in  the  formation  of  the  various  tissues  and  organs, 
and  in  applying  the  theory  of  epigenesis  to  the  cells  and  the 
tissues  they  compose,  raised  the  theory  of  germinal  layers, 
at  least  as  far  as  it  regards  the  vertebrates,  to  a  high  degree 
of  perfection. 

Remak  showed  that  three  layers  are  formed  out  of  the 
two  germinal  layers  which  compose  the  first  simple  leaf-shaped 


MODERN  EMBRYOLOGY 


structure  of  the  vertebrate  body  (or  the  "  germinal  disk  "),  as 
the  lower  layer  splits  into  two  plates.  These  three  layers 
have  a  very  definite  relation  to  the  various  tissues.  First  of 
all,  the  cells  which  form  the  outer  skin  of  the  body  (the 
epidermis),  with  its  various  dependencies  (hairs,  nails,  etc.) — 
that  is  to  say,  the  entire  outer  envelope  of  the  body — are 
developed  out  of  the  outer  or  upper  layer;  but  there  are  also 
developed  in  a  curious  way  out  of  the  same  layer  the  cells 
which  form  the  central  nervous  system,  the  brain  and  the 
spinal  cord.  In  the  second  place,  the  inner  or  lower  germinal 
layer  gives  rise  only  to  the  cells  which  form  the  epithelium 
(the  whole  inner  lining)  of  the  alimentary  canal  and  all  that 
depends  on  it  (the  lungs,  liver,  pancreas,  etc.),  or  the  tissues 
that  receive  and  prepare  the  nourishment  of  the  body.  Finally, 
the  middle  layer  gives  rise  to  all  the  other  tissues  of  the  body, 
the  muscles,  blood,  bones,  cartilage,  etc.  Remak  further 
proved  that  this  middle  layer,  which  he  calls  "  the  motor- 
germinative  layer,"  proceeds  to  sub-divide  into  two  secondary 
layers.  Thus  we  find  once  more  the  four  layers  which  Baer 
had  indicated.  Remak  calls  the  outer  secondary  leaf  of  the 
middle  layer  (Baer's  "  muscular  layer")  the  "skin  layer"  (it 
would  be  better  to  say,  skin-fibre  layer) ;  it  forms  the  outer 
wall  of  the  body  (the  true  skin,  the  muscles,  etc.).  To  the 
inner  secondary  leaf  (Baer's  "  vascular  layer ")  he  gave  the 
name  of  the  "alimentary-fibre  layer";  this  forms  the  outer 
envelope  of  the  alimentary  canal,  with  the  mesentery,  the 
heart,  the  blood-vessels,  etc. 

On  this  firm  foundation  provided  by  Remak  for  histogeny, 
or  the  science  of  the  formation  of  the  tissues,  our  knowledge 
has  been  gradually  built  up  and  enlarged  in  detail.  There 
have  been  several  attempts  to  restrict  and  even  destroy 
Remak's  principles.  The  two  anatomists,  Reichert  (of  Berlin) 
and  Wilhelm  His  (of  Leipzic),  especially,  have  endeavoured 
in  their  works  to  introduce  a  new  conception  of  the  embryonic 
development  of  the  vertebrate,  according  to  which  the  two 
primary  germinal  layers  would  not  be  the  sole  sources  of 
formation.  But  these  efforts  were  so  seriously  marred  by 
ignorance  of  comparative  anatomy,  an  imperfect  acquaintance 


MODERN  EMBRYOLOG Y 


with  ontogenesis,  and  a  complete  neglect  of  phylogenesis, 
that  they  could  not  have  more  than  a  passing  success.  We 
can  only  explain  how  these  curious  attacks  of  Reichert  and 
His  came  to  be  regarded  for  a  time  as  advances  by  the  general 
lack  of  discrimination  and  of  grasp  of  the  true  object  o( 
embryology. 

Wilhelm  His  published,  in  186S,  his  extensive  Researches 
into  the  Earliest  Form  of  the  Vertebrate  Body,1  one  of  the 
curiosities  of  embrvological  literature.  The  author  imagines 
that  he  can  build  a  "  mechanical  theory  of  embryonic  develop- 
ment "  by  merely  giving  an  exact  description  of  the  embryology 
of  the  chick,  without  any  regard  to  comparative  anatomy  and 
phylogeny,  and  thus  falls  into  an  error  that  is  almost  without 
parallel  in  the  history  of  biological  literature.  As  the  final 
result  of  his  laborious  investigations,  His  tells  us  "that  a 
comparatively  simple  law  of  growth  is  the  one  essential  thing 
in  the  first  development.  Every  formation,  whether  it  consist 
in  cleavage  of  layers,  or  folding,  or  complete  division,  is  a 
consequence  of  this  fundamental  law."  Unfortunately,  he 
does  not  explain  what  this  "  law  of  growth  "  is;  just  as  other 
opponents  of  the  theory  of  selection,  who  would  put  in  its 
place  a  great  "law  of  evolution,"  omit  to  tell  us  anything 
about  the  nature  of  this.  Nevertheless,  it  is  quite  clear  from 
His's  works  that  he  imagines  constructive  Nature  to  be  a  sort 
of  skilful  tailor.  The  ingenious  operator  succeeds  in  bringing 
into  existence,  by  "  evolution,"  all  the  various  forms  of  living 
things  by  cutting  up  in  different  ways  the  germinal  layers, 
bending  and  folding,  tugging  and  splitting,  and  so  on. 
Bending  and  folding,  especially,  play  an  important  part  in 
this  sartorial  theory  of  embryology.  "  Not  only  the  division 
of  head  from  trunk,  stem  from  periphery,  but  even  the  form 
of  the  members  and  the  separation  of  the  brain,  the  sense- 
organs,  the  primitive  vertebral  column,  the  heart,  and  the 
rudimentary  bowels,  can  be  proved  convincingly  to  be 
mechanical  consequences  of  the  first  folding  process."  The 
funniest  part  of  it  is  when  the  tailor  comes  to  fashion  the  two 

'  None  of  UN's  works  have  been  translated  into  English. 


MODERX  EMBRYOLOGY 


pairs  of  limbs:  "  The  form  is  like  the  four  corners  of  a  letter, 
obtained  by  the  crossing  of  four  folds  that  surround  the 
body."  But  this  "envelope  theory"  is  surpassed  by  the 
"  rag-bag  theory  "  with  which  His  explains  the  rudimentary 
organs:  "Organs  (such  as  the  hypophysis  or  the  thyroid 
gland)  for  which  no  physiological  function  has  yet  been 
found;  they  are  embryonic  remnants,  which  we  might  com- 
pare to  the  superfluous  pieces  that  are  left  over  when  a  coat  is 
cut  out  even  in  the  most  economic  fashion "  (!).  So  our 
Nature-tailor  now  throws  her  leavings  into  the  rag-bag.  If 
our  skull-less  ancestors  of  the  Silurian  period  had  had  any 
presentiment  of  such  vagaries  as  these  on  the  part  of  their 
human  successors,  they  would  certainly  have  preferred  to 
abandon  altogether  the  ciliated  groove  at  their  gill-openings, 
rather  than  pass  it  on  to  the  amphioxus,  and  thus  leave  us 
the  equivocal  gift  of  the  thyroid  gland  (which  becomes  the 
dreaded  goitre  when  it  is  morbidly  enlarged). 

But  the  most  important  and  extensive  of  the  embryo- 
logical  theories  of  His  was  his  famous  "  theory  of  the 
parablasts."  According  to  this,  the  human  body  (and  that  of 
all  other  vertebrates)  is  made  up  at  first  of  two  different 
organisms,  which  arise  from  two  entirely  separate  embryonic 
structures,  the  chief  embryo  and  the  secondary  embryo.  It 
is  only  the  chief  embryo,  or  the  "  Archiblast,"  that  developes 
from  the  fertilised  ovum,  and  is  built  from  the  two  primary 
germinal  layers  which  are  formed  by  its  repeated  sub-division. 
On  the  other  hand,  the  secondary  embryo,  or  the  "  Para- 
blast,"  is  formed,  not  out  of  the  germinal  layers,  but  from 
parts  of  the  white  yelk ;  the  cells  which  compose  it  come 
from  the  follicle-cells  of  the  membrana  granulosa,  and  have 
passed  from  the  ovary  into  the  yelk.  Hence  the  parablast  is 
an  additional  gift  from  the  mother,  the  archiblast  alone 
coming  from  both  parents,  as  a  product  of  the  fertilised 
ovum,  and  transmitting  their  features  to  the  offspring. 
From  this  secondary  embryo  are  developed  (partheno- 
genetically)  the  tissues  of  the  blood-vessels  and  the  connec- 
tive parts  (bones,  cartilages,  etc.)  ;  while  all  the  other  tissues 
of  the  vertebrate  body  are  formed  from  the  sexually-produced 


MODEKX  EM  II  MYOLOGY 


chief  embryo.  The  two  embryos  are  at  first  quite  indepen- 
dent, "  sharply  distinguished,  not  only  in  regard  to  origin, 
but  also  from  the  histological  and  physiological  points  of 
view."  Thus  the  vertebrate  organism  is  a  double  being, 
formed  by  the  "  symbiosis,"  or  the  gradual  coalescence,  of  two 
animals  that  were  at  first  distinct.  As  the  lichen  is  made  up 
of  two  distinct  plants,  a  fungus  and  an  alga,  so,  according  to 
His,  every  vertebrate  is  composed  of  two  separate  animals, 
an  archiblast  and  a  parablast.  I  have  pointed  out  in  my 
essay  on  The  Origin  and  Development  of  the  Animal 
Tissues  (1884)  the  far-reaching  consequences  that  would 
follow  from  this  "symbiosis  of  the  vertebrate." 

This  parablast  theory,  like  His's  other  embryological 
theories,  excited  a  good  deal  of  interest  at  the  time  of  its 
publication,  and  has  evoked  a  fair  amount  of  literature  in  the 
last  few  decades.  His  professed  to  explain  the  most  com- 
plicated parts  of  organic  construction  (such  as  the  develop- 
ment of  the  brain)  in  the  simplest  way  on  mechanical 
principles,  and  to  derive  them  immediately  from  simple 
physical  processes  (such  as  unequal  distribution  of  strain  in 
an  elastic  plate).  It  is  quite  true  that  a  mechanical  or 
monistic  explanation  (or  a  reduction  of  natural  phenomena 
to  physical  and  chemical  processes)  is  the  ideal  of  modern 
science,  and  this  ideal  would  be  realised  if  we  could  succeed 
in  expressing  these  formative  processes  in  mathematical 
formulae.  His  has,  therefore,  inserted  plenty  of  numbers  and 
measurements  in  his  embryological  works,  and  given  them 
an  air  oi  "exact"  scholarship  by  putting  in  a  quantity  of 
mathematical  tables.  Unfortunately,  they  are  of  no  value, 
and  do  not  help  us  in  the  least  in  forming  an  "exact" 
acquaintance  with  the  embryonic  phenomena.  Indeed,  they 
wander  from  the  true  path  altogether  by  neglecting  the 
phylogenetic  method;  this,  he  thinks,  is  "a  mere  by-path," 
and  is  "  not  necessary  at  all  for  the  explanation  of  the  facts  ol 
embryology,"  which  are  the  direct  consequence  of  physio- 
logical principles.  What  His  takes  to  be  a  simple  physical 
process — for  instance,  the  folding  o(  the  germinal  layers  (in 
the  formation  of  the  medullary  tube,  alimentary  tube,  etc.) — 


52  MODERN  EMBRYOLOGY 

is,  as  a  matter  of  fact,  the  direct  result  of  the  growth  of  the 
various  cells  which  form  those  organic  structures  ;  but  these 
growth-motions  have  themselves  been  transmitted  by  heredity 
from  parents  and  ancestors,  and  are  only  the  hereditary 
repetition  of  countless  phylogenetic  changes  which  have 
taken  place  for  thousands  of  years  in  the  race-history  of  the 
said  ancestors. 

Each  of  these  historical  changes  was,  of  course,  originally 
due  to  adaptation  ;  it  was,  in  other  words,  physiological,  and 
reducible  to  mechanical  causes.  But  we  have,  naturally,  no 
means  of  observing  them  now.  It  is  only  by  the  hypotheses 
of  the  science  of  evolution  that  we  can  form  an  approximate 
idea  of  the  organic  links  in  this  historic  chain.  I  have 
contrasted  these  phylogenetic  theories  with  the  pseudo- 
mechanical  theories  of  His  in  my  essay  on  The  Aims  and 
Methods  of  Modern  Embryology  (1875).  I  have  also  given  in 
this  essay  a  criticism  of  the  curious  theories  of  evolution 
which  Alexander  Goette  has  put  forward  in  his  compre- 
hensive and  finely  illustrated  study  (1875)  of  the  develop- 
ment of  the  ringed-snake  ;  and  of  the  religious  and  mystic 
views  of  Louis  Agassiz.  Such  vagaries  as  these  are  scarcely 
possible  in  any  other  science  to-day.  That  they  crop  up  in 
the  science  of  embryology  is  due  in  part  to  the  extreme 
difficulty  and  intricacy  of  its  object,  and  in  part  to  the 
inadequate  training  of  many  of  the  workers  in  this  field.  In 
fine,  it  is  worth  noting  that,  though  His's  pseudo-mechanical 
method  has  (like  the  very  different  method  of  Goette)  been 
much  admired,  it  has  not  been  developed  or  applied  with  any 
success  by  any  other  scientist.  No  results  of  any  value  have 
been  attained  by  it. 

All  the  best  recent  research  in  animal  embryology  has  led 
to  the  confirmation  and  development  of  Baer  and  Remak's 
theory  of  the  germinal  layers.  One  of  the  most  important 
advances  in  this  direction  of  late  was  the  discovery  that  the 
two  primary  layers  out  of  which  is  built  the  body  of  all 
vertebrates  (including  man)  are  also  present  in  all  the 
invertebrates,  with  the  sole  exception  of  the  lowest  group,  the 
unicellular    protozoa.     Huxley   had    detected    them    in    the 


MODERX  EM/iR  1  OLOG 1 ' 


medusa  in  1S49.  lie  showed  that  the  two  layers  of  cells 
from  which  the  body  of  this  zoophyte  is  developed 
correspond,  both  morphologically  and  physiologically,  to  the 
two  original  germinal  layers  of  the  vertebrate.  The  outer 
layer,  from  which  come  the  external  skin  and  the  muscles, 
was  then  called  by  Allman  (1853)  the  "ectoderm"  (  outer 
layer,  or  skin)  ;  the  inner  layer,  which  forms  the  alimentary 
and  reproductory  organs,  was  called  the  "entoderm"  (=  inner 
layer).  In  1867  and  the  following  years  the  discovery  of  the 
germinal  layers  was  extended  to  other  groups  of  the  inverte- 
brates. In  particular,  the  indefatigable  Russian  zoologist, 
Kowalevsky,  found  them  in  all  the  most  diverse  sections 
of  the  invertebrates — the  worms,  tunicates,  echinoderms, 
molluscs,  articulates,  etc. 

In  my  monograph  on  the  sponges  (1872)  I  myself  proved 
that  these  two  primary  germinal  layers  are  also  found  in  that 
group,  and  that  they  may  be  traced  from  it  right  up  to  man, 
through  all  the  various  classes,  in  analogous  (or  homologous) 
form.  This  "  homology  of  the  two  primary  germinal  layers  " 
extends  through  the  whole  of  the  metazoa,  or  tissue-forming 
animals  ;  that  is  to  say,  through  the  whole  animal  kingdom, 
with  the  one  exception  of  its  lowest  section,  the  unicellular 
beings,  or  protozoa.  These  lowly  organised  animals  do  not 
form  germinal  layers,  and  therefore  do  not  succeed  in 
forming  true  tissue.  Their  whole  body  consists  of  a  single 
cell  (as  is  the  case  with  the  amceba;  and  infusoria),  or  of  a 
loose  aggregation  of  only  slightly  differentiated  cells,  though 
it  may  not  even  reach  the  full  structure  of  a  single  cell  (as 
with  the  monera).  But  in  all  other  animals  the  ovum  first 
grows  into  two  primary  layers,  the  outer  or  animal  layer  (the 
ectoderm,  epiblast,  or  ectoblast),  and  the  inner  or  vegetal 
layer  (the  entoderm,  hvpoblast,  or  endoblast)  ;  and  from  these 
the  tissues  and  organs  are  formed.  The  first  and  oldest 
organ  of  all  these  metazoa  is  the  primitive  gut  (or  progaster) 
and  its  opening,  the  primitive  mouth  (prostoma).  The 
typical  embryonic  form  of  the  metazoa,  as  it  is  presented 
for  a  time  by  this  simple  structure  of  the  two-layered  body,  is 
called  the  gastrula  ;   it  is  to  be  conceived  as  the   hereditary 


MODER.X  EM  BR  5  'OLOGY 


reproduction  of  some  primitive  common  ancestor  of  the 
metazoa,  which  we  call  the  gastrcea.  This  applies  to  the 
sponges  and  other  zoophyta,  and  to  the  worms,  the 
mollusca,  echinoderma,  articulata,  and  vertebrata.  All  these 
animals  may  be  comprised  under  the  general  heading  of 
"gut  animals,"  or  metazoa,  in  contradistinction  to  the  gut- 
less protozoa. 

I  have  pointed  out  in  my  Study  of  the  Gastrcea  Theory  [not 
translated]  (1873)  the  important  consequences  of  this  concep- 
tion in  the  morphology  and  classification  of  the  animal  world. 
I  also  divided  the  realm  of  metazoa  into  two  great  groups,  the 
lower  and  higher  metazoa.  In  the  first  are  comprised  the 
ccelenterata  (also  called  zoophytes,  or  "  plant-animals  ").  In 
the  lower  forms  of  this  group  the  body  consists  throughout 
life  merely  of  the  primary  germinal  layers,  with  the  cells 
sometimes  more  and  sometimes  less  differentiated  ;  this  is  the 
case  with  the  gastrasads,  the  simpler  sponges  (protospongia), 
the  hydropolyps,  and  the  lower  medusje.  But  with  the  higher 
forms  of  the  ccelenterata  (the  corals,  higher  medusa;,  cteno- 
phora,  and  platodes)  a  middle  layer,  or  mesoderm,  often  of 
considerable  size,  is  developed  between  the  other  two  layers  ; 
but  blood  and  an  internal  cavity  are  still  lacking. 

To  the  second  great  group  of  the  metazoa  I  gave  the  name 
of  the  coelomaria,  or  bilaterata  (or  the  bilateral  higher  forms). 
They  all  have  a  cavity  within  the  body  (cceloma),  and  most 
of  them  have  blood  and  blood-vessels.  In  this  are  comprised 
the  six  higher  stems  of  the  animal  kingdom,  the  annulata 
and  their  descendants,  the  mollusca,  echinoderma,  articulata, 
tunicata,  and  vertebrata.  In  all  these  bilateral  organisms  the 
two-sided  body  is  formed  out  of  four  secondary  germinal 
layers,  of  which  the  inner  two  construct  the  wall  of  the 
alimentary  canal,  and  the  outer  two  the  wall  of  the  body. 
Between  the  two  pairs  of  layers  lies  the  cavity  (cceloma). 

Although  I  laid  special  stress  on  the  great  morphological 
importance  of  this  cavity  in  my  Study  of  the  Gastrcea  Theory, 
and  endeavoured  to  prove  the  significance  of  the  four  secondary 
germinal  layers  in  the  organisation  of  the  ccelomaria,  I  was 
unable  to  deal  satisfactorily  with  the  difficult  question  of  the 


MODERN  EM  UK  YOI.OCY 


mode  of  their  origin.  This  was  done  eight  years  afterwards 
by  the  brothers  Oscar  and  Richard  Hertwig  in  their  careful 
and  extensive  comparative  studies.  In  their  masterly  Cesium 
Theory:  An  Attempt  to  Explain  the  Middle  Germinal  Layer 
[not  translated]  (1881)  they  showed  that  in  most  of  the 
metazoa.especially  in  all  the  vertebrates,  the  body-cavity  arises 
in  the  same  way,  by  the  turning  up  of  two  of  the  entoderm 
sacs.  These  two  ccelum-pouehes  grow  out  from  the  rudi- 
mentary mouth  of  the  gastrula,  between  the  two  primary 
layers.  The  inner  plate  of  the  two-layered  ccelum-pouch 
(the  visceral  layer)  joins  itself  to  the  entoderm ;  the  outer 
plate  (parietal  layer)  unites  with  the  ectoderm.  Thus  are 
formed  the  double-layered  gut-wall  within  and  the  double- 
layered  body-wall  without;  and  between  the  two  is  formed 
the  cavity  of  the  ccelum,  by  the  blending  of  the  right  and  left 
cojlum-sacs. 

The  many  new  points  of  view  and  fresh  ideas  suggested 
by  my  gastnea  theory  and  Hertwig's  ccelum  theorv  led  to  the 
publication  of  a  number  of  writings  on  the  theory  of  germinal 
layers.  Most  of  them  set  out  to  oppose  it  at  first,  but  in  the 
end  the  majority  supported  it.  Of  late  years  both  theories 
are  accepted  in  their  essential  features  by  nearly  every  com- 
petent man  of  science,  and  light  and  order  have  been  intro- 
duced into  this  once  dark  and  contradictory  field  of  research. 
A  further  cause  of  congratulation  for  this  solution  of  the 
great  embryological  controversy  is  that  it  brought  with  it  a 
recognition  of  the  need  for  phylogenetic  Study  and  expla- 
nation. 

Interest  and  practice  in  embryological  research  have  been 
remarkably  stimulated  during  the  past  thirty  years  by  this 
appreciation  of  phylogenetic  methods.  Hundreds  of  assiduous 
and  able  observers  are  now  engaged  in  the  development  of 
comparative  embryology  and  its  establishment  on  a  basis  of 
evolution,  whereas  they  numbered  only  a  few  dozen  not  many 
decades  ago.  It  would  take  too  long  to  enumerate  even 
the  most  important  of  the  countless  valuable  works  which 
have  enriched  embryological  literature  since  that  time. 
References   to   them    will   be   found    in    the   latest  manuals  of 


56  MODERN  EMBRYOLOGY 

embryology   of  Kolliker,    Balfour,   Hertwig,   Kollman,  Kor- 
schelt,  and  Heider. 

Kolliker's  Entwickelitngsgeschichte  des  Menschen  und  der 
hbherer  Thiere,  the  first  edition  of  which  appeared  forty-two 
years  ago,  had  the  rare  merit  at  that  time  of  gathering  into 
presentable  form  the  scattered  attainments  of  the  science,  and 
expounding  them  in  some  sort  of  unity  on  the  basis  of  the 
cellular  theory  and  the  theory  of  germinal  layers.  Unfortu- 
nately, the  distinguished  Wiirtzburg  anatomist,  to  whom 
comparative  anatomy,  histology,  and  ontogeny  owe  so  much, 
is  opposed  to  the  theory  of  descent  generally  and  to  Darwinism 
in  particular.  In  the  latest  edition  of  his  work  (1884)  he 
rejected  the  evolutionary  significance  of  the  facts  of  embryo- 
logy, as  I  pointed  it  out,  and  the  gastraea  theory.  On  the 
other  hand,  he  subscribes  (though  less  fully  of  late  years)  to 
the  theories  of  His,  and  has  contributed  a  good  deal  by  his 
great  authority  to  the  prestige  they  enjoyed  for  a  time. 

All  the  other  manuals  I  have  mentioned  take  a  decided 
stand  on  evolution.  Francis  Balfour  has  carefully  collected 
and  presented  with  discrimination,  in  his  Manual  of  Compara- 
tive Embryology  (1880),  the  very  scattered  and  extensive  litera- 
ture of  the  subject ;  he  has  also  widened  the  basis  of  the  gastrasa 
theory  by  a  comparative  description  of  the  rise  of  the  organs 
from  the  germinal  layers  in  all  the  chief  groups  of  the  animal 
kingdom,  and  has  given  a  most  thorough  empirical  support 
to  the  principles  I  have  formulated.  A  comparison  of  his 
work  with  the  excellent  Text-book  of  the  Embryology  of  the 
Vertebrates  (1890)  [translation,  1895]  of  Korschelt  and  Heider 
shows  what  astonishing  progress  has  been  made  in  the  science 
in  the  course  of  ten  years.  I  would  especially  recommend 
the  manuals  of  Julius  Kollman  and  Oscar  Hertwig  to  those 
readers  who  are  stimulated  to  further  study  by  these  chapters 
on  human  embryology.  Kollmann's  Lehrbuch  der  entwicke- 
lungsgeschichte  des  Menschen  (1898)  is  commendable  for  its 
clear  treatment  of  the  subject  and  very  fine  original  illustra- 
tions; its  author  adheres  firmly  to  the  biogenetic  law,  and  uses 
it  throughout  with  considerable  profit.  That  is  not  the  case 
in   Oscar  Hertwig's   recent    Text-book  of  the  Embryology  of 


vo/ >/:a:y  i:\iiiryology 


Man  and  the  Mammals  [translations  [892  and  1899)  (seventh 
edition,  1902).  This  ahle  anatomist  lias  of  late  often  been 
quoted  as  an  opponent  of  the  biogenetic  law,  although  he 
himself  had  demonstrated  its  great  value  thirty  years  ago  in 
his  Untersuchungen  uber  Bait  und  Entwickelung  der  Plakoid- 
schuppen.  His  recent  vacillation  is  partly  due  to  the  timidity 
which  our  "exact  "  scientists  have  with  regard  to  hypotheses; 
though  it  is  quite  impossible  to  make  any  headway  in  the 
explanation  of  facts  without  them.  However,  the  purely 
descriptive  part  of  embrvologv  in  Hertwig's  Text-book  is  very 
thorough  and  reliable.  A  shorter  account  is  given  in  his 
Elemente  der  Entwickelungslehre  (Jena,  1900),  and  a  very 
good  summary  of  special  work  done  by  many  authors  in  his 
Handbuch  der  vergleichenden  und  experimentellen  Entwicke- 
lungslehre der  Wirbelthiere  (Jena,  1901). 

A  new  branch  of  embrvological  research  has  been  studied 
very  assiduously  in  the  last  decade  of  the  nineteenth  century — 
namely,  "  experimental  embryology."  The  great  importance 
which  has  been  attached  to  the  application  of  physical 
experiments  to  the  living  organism  for  the  last  hundred 
years,  and  the  valuable  results  that  it  has  given  to  physiology 
in  the  study  of  the  vital  phenomena,  have  led  to  its  extension 
to  embryology.  I  was  the  first  to  make  experiments  of  this 
kind  during  a  stay  of  four  months  on  the  Canary  Island, 
Lanzerote,  in  1866.  I  there  made  a  thorough  investigation 
of  the  almost  unknown  embryology  of  the  siphonophora.  1 
cut  a  number  of  the  embryos  of  these  animals  (which  develop 
freely  in  the  water,  and  pass  through  a  very  curious  transfor- 
mation), at  an  early  stage,  into  several  pieces,  and  found 
that  a  fresh  organism  (more  or  less  complete,  according  to 
the  size  of  the  piece)  was  developed  from  each  particle.  I 
have  given  illustrations  of  the  curious  larva  (sometimes  of 
quite  monstrous  shapes)  which  form  from  them  on  plates 
11-14  °f  m.v  Entwickelungsgeschichte  der  Siphonophoren 
(Utrecht,  1869). 

More  recently  some  of  my  pupils  have  made  similar 
experiments  with  the  embryos  of  vertebrates  (especially  the 
frog)  and    some   of    the    invertebrates.      Wilhelm    Roux,    in 


SS  MODERN  EM  BR  YOLOG  Y 

particular,  has  made  extensive  experiments,  and  based  on 
them  a  special  "mechanical  embryology,"  which  has 
given  rise  to  a  good  deal  of  discussion  and  controversy. 
Roux  has  published  a  special  journal  for  these  subjects  since 
1895,  the  Archiv  fur  Entmickelungsmechanik.  The  contri- 
butions to  it  are  very  varied  in  value.  Many  of  them  are 
valuable  papers  on  the  physiology  and  pathology  of  the 
embryo.  Pathological  experiments — the  placing  of  the 
embryo  in  abnormal  conditions — have  yielded  many 
interesting  results  ;  just  as  the  physiology  of  the  normal 
body  has  for  a  long  time  derived  assistance  from  the 
pathology  of  the  diseased  organism.  Other  of  these 
mechanical-evolutionary  articles  return  to  the  erroneous 
methods  of  His,  and  are  only  misleading.  This  must  be 
said  of  the  many  contributions  of  mechanical  embryology 
which  take  up  a  position  of  hostility  to  the  theory  of  descent 
and  its  chief  embryological  foundation — the  biogenetic  law. 
This  law,  however,  when  rightly  understood,  is  not  opposed 
to,  but  is  the  best  and  most  solid  support  of,  a  sound 
mechanical  embryology.  Impartial  reflection  and  a  due 
attention  to  paleontology  and  comparative  anatomy  should 
convince  these  one-sided  mechanicists  that  the  facts  they 
have  discovered — and,  indeed,  the  whole  embryological 
process — cannot  be  fully  understood  without  the  theory  of 
descent  and  the  biogenetic  law. 


CHAPTER   IV. 

THE  OLDER  PHYLOGENY1 

Evolution  before  Darwin.  The  origin  of  species.  Carl  Linne  gives  a  defini- 
tion of  species  and  genus,  and  associates  it  with  the  Biblical  story  of 
creation.  The  deluge.  Paleontology.  The  catastrophic  theory  of 
Georges  Cuvier.  Repeated  revolutions  on  earth  and  fresh  creations. 
Lyell's  theory  of  continuity.  The  natural  causes  of  the  gradual  formation 
of  the  earth.  Supernatural  origin  of  living  things.  Dualistic  natural 
philosophy  of  Immanuel  Kant.  Monistic  natural  philosophy  of  Jean 
Lamarck.  His  life.  His  Philosophic  Zoologique.  The  first  scientific 
treatment  of  evolution.  Transformation  of  organs  by  use  and  habit, 
together  with  heredity.  Application  of  the  theory  to  man.  Descent  of 
man  from  the  ape.  Wolfgang  Goethe.  His  scientific  studies.  His 
morphology.  His  studies  on  the  formation  and  transformation  of  organic 
natures.  Goethe's  theory  of  the  impulse  to  specification  (heredity)  and 
metamorphosis  (adaptation). 

Tin-:  embryology  of  man  and  the  animals,  the  historv  of 
which  we  have  reviewed  in  the  last  two  chapters,  was 
mainly  a  descriptive  science  forty  years  ago.  The  earlier 
investigations  in  this  province  were  chiefly  directed  to  the 
discovery,  by  careful  observation,  of  the  wonderful  facts  of 
the  embryonic  development  of  the  animal  body  from  the 
ovum.  Forty  years  ago  no  one  dared  attack  the  question  of 
the  causes  of  these  phenomena.  For  fully  a  century,  from 
the  year  1759,  when  Wolffs  solid  Theoria  generation  is 
appeared,  until  1859,  when  Darwin  published  his  famous 
Origin  of  Species,  the  real  causes  of  the  embryonic  processes 
were  quite  unknown.  No  one  thought  of  seeking  the 
agencies  that  effected  this  marvellous  succession  of  struc- 
tures. The  task  was  thought  to  be  so  difficult  as  almost  to 
pass  beyond  the  limits  of  human  thought.  It  was  reserved 
for  Charles  Darwin  to  initiate  us  into  the  knowledge  of  these 
causes.  This  compels  us  to  recognise  in  this  great  genius, 
who  wrought  a    complete    revolution    in    the    whole    field    of 

1  Cf.   Clodd's  Pioneers  "/   Evolution  and    Packard's    Lamarck  ami  X,-.- 
Lamarckism  and  Lamarck  the  Founder  of  Evolution. 


THE  OLDER  PHYLOGENY 


biology,  a  founder  at  the  same  time  of  a  new  period  in 
embryology.  It  is  true  that  Darwin  occupied  himself  very 
little  with  direct  embryological  research,  and  even  in  his 
chief  work  he  only  touches  incidentally  on  the  embryonic 
phenomena ;  but  by  his  reform  of  the  theory  of  descent 
and  the  founding  of  the  theory  of  selection  he  has  given 
us  the  means  of  attaining  to  a  real  knowledge  of  the 
causes  of  embryonic  formation.  That  is,  in  my  opinion, 
the  chief  feature  in  Darwin's  incalculable  influence  on  the 
whole  science  of  evolution. 

When  we  turn  our  attention  to  this  latest  period  of 
embryological  research,  we  pass  into  the  second  division  of 
organic  evolution — stem-evolution,  or  phylogeny.  I  have 
already  indicated  in  the  first  chapter  the  important  and 
intimate  causal  connection  between  these  two  sections  of  the 
science  of  evolution — between  the  evolution  of  the  individual 
and  that  of  his  ancestors.  We  have  formulated  this  connec- 
tion in  the  biogenetic  law;  the  shorter  evolution,  that  of  the 
individual,  or  ontogenesis,  is  a  rapid  and  summary  repetition, 
a  condensed  recapitulation,  of  the  larger  evolution,  or  that  of 
the  species.  In  this  principle  we  express  all  the  essential 
points  relating  to  the  causes  of  evolution ;  and  we  shall  seek 
throughout  this  work  to  confirm  this  principle  and  lend  it  the 
support  of  facts.  When  we  look  to  its  causal  significance, 
perhaps  it  would  be  better  to  formulate  the  biogenetic  law 
thus:  "The  evolution  of  the  species  and  the  stem  (phvlon) 
shows  us,  in  the  physiological  functions  of  heredity  and 
adaptation,  the  conditioning  causes  on  which  the  evolution  of 
the  individual  depends";  or,  more  briefly:  "  Phylogenesis  is 
the  mechanical  cause  of  ontogenesis." 

We  owe  it  to  Darwin  that  we  are  now  in  a  position  to  trace 
and  appreciate  these  hitherto  obscure  causes  of  embryonic 
development,  and  so  we  give  his  name  to  a  new  period  in 
embryology.  But  before  we  examine  the  great  achievement 
by  which  Darwin  revealed  the  causes  of  evolution  to  us,  we 
must  glance  at  the  efforts  of  earlier  scientists  to  attain  this 
object.  Our  historical  inquiry  into  these  will  be  even  shorter 
than  that  into  the  work  done  in  the  field  of  ontogeny.     We 


THE  OLDER  PHYLOGENY 


have  very  few  names  to  consider  here.  At  the  head  of  them 
we  find  the  great  French  naturalist,  Jean  Lamarck,  who  first 
established  evolution  as  a  scientific  theory  in  1809.  Even 
before  his  time,  however,  the  chief  philosopher,  Kant,  and 
the  chief  poet,  Goethe,  of  Germany  had  occupied  themselves 
with  the  subject.  But  their  efforts  passed  almost  without 
recognition  in  the  eighteenth  century.  A  "philosophy  of 
nature  "did  not  arise  until  the  beginning  of  the  nineteenth 
century.  In  the  whole  of  the  time  before  this  no  one  had 
ventured  to  raise  seriously  the  question  of  the  origin  of 
species,  which  is  the  culminating  point  of  phylogeny.  On 
all  sides  it  was  regarded  as  an  insoluble  enigma. 

The  whole  science  of  the  evolution  of  man  and  the  other 
animals  is  intimately  connected  with  the  question  of  the 
nature  of  species,  or  with  the  problem  of  the  origin  of  the 
various  animals  which  we  group  together  under  the  name  of 
species.  Thus  the  definition  of  the  species  becomes  impor- 
tant. It  is  well  known  that  this  definition  was  given  by 
Linne,  who,  in  his  famous  Systema  Natures  (1735),  was  the 
first  to  classify  and  name  the  various  groups  of  animals  and 
plants,  and  drew  up  an  orderly  scheme  of  the  species  then 
known.  Since  that  time  "species"  has  been  the  most 
important  and  indispensable  idea  in  descriptive  natural 
history,  in  zoological  and  botanical  classification  ;  although 
there  have  been  endless  controversies  as  to  its  real  meaning. 

What,  then,  is  this  "organic  species"?  Linne  himself 
did  not  give  a  very  clear  account  of  it.  He  unfortunately 
relied  on  religious  notions  which  the  dominant  creed  had 
founded  on  the  Mosaic  story  of  creation,  and  which  have  not 
vet  wholly  disappeared.  Linne,  in  fact,  appealed  directly  to 
the  Mosaic  narrative;  he  believed  that,  as  it  is  stated  in 
Genesis,  one  pair  of  each  species  of  animals  and  plants  was 
created  in  the  beginning,  and  that  all  the  individuals  of  each 
species  are  the  descendants  of  these  created  couples.  As  for 
the  hermaphrodites  (organisms  that  have  male  and  female 
organs  in  one  being),  he  thought  it  sufficed  to  assume  the 
creation  of  one  sole  individual,  since  this  would  be  fully 
competent    to    propagate    its    species.     Further    developing 


THE  OLDER  PHYLOGEXY 


these  mystic  ideas,  Linne  went  on  to  borrow  from  Genesis 
the  account  of  the  deluge  and  of  Noah's  ark  as  a  ground  for 
the  chorology  of  organisms — that  is  to  say,  for  a  science 
of  their  geographical  and  topographical  distribution.  He 
accepted  the  story  that  all  the  plants,  animals,  and  men  on 
the  earth  were  swept  away  in  a  universal  deluge,  except  the 
couples  preserved  with  Noah  in  the  ark,  and  ultimately 
landed  on  Mount  Ararat.  This  mountain  seemed  to  Linne 
particularly  suitable  for  the  landing,  as  it  reaches  a  height  of 
more  than  16,000  feet,  and  thus  provides  in  its  higher  zones 
the  several  climates  demanded  by  the  various  species  of 
animals  and  plants  :  the  animals  that  were  accustomed  to  a 
cold  climate  could  remain  at  the  summit ;  those  used  to  a 
warm  climate  could  descend  to  the  foot ;  and  those  requiring 
a  temperate  climate  could  remain  half-way  down.  From  this 
point  the  re-population  of  the  earth  with  animals  and  plants 
could  proceed. 

It  was  impossible  to  have  any  scientific  notion  of  the 
method  of  evolution  in  Linne's  time,  as  one  of  the  chief 
sources  of  information,  paleontologv,  was  still  wholly 
unknown.  This  science  of  the  fossil  remains  of  extinct 
animals  and  plants  is  very  closely  bound  up  with  the  whole 
question  of  evolution.  It  is  impossible  to  explain  the  origin 
of  living  organisms  without  appealing  to  it.  But  this  science 
did  not  rise  until  a  much  later  date.  The  real  founder  of 
scientific  paleontology  was  Georges  Cuvier,  the  most  distin- 
guished zoologist  who,  after  Linne,  worked  at  the  classifi- 
cation of  the  animal  world,  and  effected  a  complete  revolution 
in  systematic  zoology  at  the  beginning  of  the  nineteenth 
century.  The  influence  of  this  famous  scientist,  which  was 
of  extraordinary  service,  especially  in  the  first  three  decades 
of  the  century,  was  so  great  that  he  opened  up  new  paths 
in  nearly  every  part  of  scientific  zoology,  particularly  in 
classification,  comparative  anatomy,  and  paleontology.  It 
is  important,  therefore,  to  inquire  what  idea  Cuvier  had  of 
the  nature  of  the  species.  In  this  respect  he  associated 
himself  with  Linne  and  the  Mosaic  story  of  creation,  though 
this  was  more   difficult  for  him  with   his  acquaintance  with 


THE  OLDER  PHYLOGENY  63 

fossil  remains.  He  clearly  showed  that  a  number  of  quite 
different  animal  populations  have  lived  on  the  earth  ;  and  he 
claimed  that  we  must  distinguish  a  number  of  stages  in  the 
history  of  our  planet,  each  of  which  was  characterised  by  a 
special  population  of  animals  and  plants. 

Cuvier  had,  naturally,  to  meet  the  question  of  the  origin 
of  these  different  populations,  and  if  they  were  connected 
with  each  other  or  not.  He  answered  this  question  in  the 
negative,  affirming  that  the  successive  populations  were 
quite  independent  of  each  other,  and  that  therefore  the  super- 
natural creative  act,  which  was  demanded  as  the  origin  of  the 
animals  and  plants  by  the  dominant  creed,  must  have  been 
repeated  several  times.  In  this  way  a  whole  series  of 
different  creative  periods  must  have  succeeded  each  other ; 
and  in  connection  with  these  he  had  to  assume  that 
stupendous  revolutions  or  cataclysms — something  like  the 
legendary  deluge — must  have  taken  place  repeatedly.  Cuvier 
was  all  the  more  interested  in  these  catastrophes  or  cata- 
clysms as  geology  was  just  beginning  to  assert  itself,  and 
great  progress  was  being  made  in  our  knowledge  of  the 
structure  and  formation  of  the  earth's  crust.  The  various 
strata  of  the  crust  were  being  carefully  examined,  especially 
by  the  famous  geologist  Werner  and  his  school,  and  the 
fossils  found  in  them  were  being  classified  ;  and  these 
researches  also  seemed  to  point  to  a  variety  of  creative 
periods.  In  each  period  the  earth's  crust,  composed  of  the 
various  strata,  seemed  to  be  differently  constituted,  just  like 
the  population  of  animals  and  plants  that  then  lived  on  it. 
Cuvier  combined  this  notion  with  the  results  of  his  own 
paleontological  and  zoological  research;  and  in  his  effort  to 
get  a  consistent  view  o(  the  whole  process  of  the  earth's 
history  he  came  to  form  the  theory  which  is  known  as  "the 
catastrophic  theory,"  or  the  theory  ot  terrestrial  revolutions. 
According  to  this  theory,  there  have  been  a  series  of  mighty 
cataclysms  on  the  earth,  and  these  have  suddenly  destroyed 
the  whole  animal  and  plant  population  then  living  on  it  ; 
after  each  cataclysm  there  was  a  fresh  creation  o\  living 
things  throughout  the  earth.     As  this  creation  could  not  be 


64  THE  OLDER  PHYLOGENY 


explained  by  natural  laws,  it  was  necessary  to  appeal  to  an 
intervention  on  the  part  of  the  Creator.  This  catastrophic 
theory,  which  Cuvier  described  in  a  special  work,  was  soon 
generally  accepted,  and  retained  its  position  in  biology  for 
half  a  century. 

However,  Cuvier's  theory  was  completely  overthrown 
sixty  years  ago  by  the  geologists,  led  by  Charles  Lyell,  the 
most  distinguished  worker  in  this  field  of  science.  Lyell 
proved  in  his  famous  Principles  of  Geology  (1830)  that  the 
theory  was  false,  in  so  far  as  it  concerned  the  crust  of  the 
earth;  that  it  was  totally  unnecessary  to  bring  in  supernatural 
agencies  or  general  catastrophes  in  order  to  explain  the 
structure  and  formation  of  the  mountains;  and  that  we  can 
explain  them  by  the  familiar  agencies  which  are  at  work 
to-day  in  altering  and  reconstructing  the  surface  of  the  earth. 
These  causes  are — the  action  of  the  atmosphere  and  water  in 
its  various  forms  (snow,  ice,  fog,  rain,  the  wear  of  the  river, 
and  the  stormy  ocean),  and  the  volcanic  action  which  is 
exerted  by  the  glowing  central  mass.  Lyell  convincingly 
proved  that  these  natural  causes  are  quite  adequate  to  explain 
every  feature  in  the  build  and  formation  of  the  crust.  Hence 
Cuvier's  theory  of  cataclysms  was  very  soon  driven  out  of  the 
province  ot  geology. 

Nevertheless,  the  theory  remained  for  another  thirty  years 
in  undisputed  authority  in  biology.  All  the  zoologists  and 
botanists  who  gave  any  thought  to  the  question  of  the  origin 
of  organisms  adhered  to  Cuvier's  erroneous  idea  of  revolutions 
and  new  creations.  It  is  one  of  the  most  curious  instances  on 
record  of  two  cognate  sciences  pursuing  for  some  time  totally 
different  ways  from  each  other.  Biology  lagged  behind  on 
the  paths  of  dualism,  and  declared  it  impossible  to  solve  the 
problem  of  the  formation  of  species  on  natural  principles; 
geology,  on  the  contrary,  advanced  rapidly  along  the  monistic 
path,  and  solved  the  problem  by  the  indication  of  the  natural 
agencies  at  work. 

In  order  to  illustrate  the  complete  stagnancy  of  biology 
from  1830  to  1859,  on  the  question  of  the  origin  of  organisms, 
or  the  formation  of  the  various  species  of  animals  and  plants, 


THE  OLDER  PHYLOGENY  65 


I  may  say,  from   my  own  experience,  that  during  the  whole 

of  my  university  studies  I  never  heard  a  single  word  said 
about  this  most  important  problem  o(  the  science.  I  was 
fortunate  enough  at  that  time  (1852  [857)  to  have  the  most 
distinguished  masters  for  every  branch  o(  biological  science. 

Not  one  o(  them  ever  mentioned  this  question  o(  the  origin 
o(  speeies.  Not  a  word  was  ever  said  about  the  earlier  efforts 
to  understand  the  formation  of  living  things,  nor  about 
Lamarck's  Philosophic  Zoologique  which  had  made  a  fresh 
attack  on  the  problem  in  1S09.  Hence  it  is  easy  to  under- 
stand the  enormous  opposition  that  Darwin  encountered  when 
he  took  up  the  question  for  the  first  lime.  His  views  seemed 
to  float  in  the  air,  without  a  single  previous  effort  to  support 
them.  The  whole  question  o(  the  formation  of  living  things 
was  considered  by  biologists,  until  1859,  as  pertaining  to  the 
province  of  religion  and  transcendentalism;  even  in  specula- 
tive philosophy,  in  which  the  question  had  been  approached 
from  various  sides,  no  one  had  ventured  to  give  it  serious 
treatment. 

This  last  circumstance  was  due  to  the  dualistic  system  of 
Immanuel  Kant,  and  the  enormous  influence  of  this  most 
important  of  recent  thinkers  down  to  our  own  time.  Kant, 
a  genius  both  in  science  and  philosophy,  taught  a  natural 
system  oi  evolution  as  far  as  the  inorganic  world  was  con- 
cerned ;  but,  on  the  whole,  adopted  a  supernaturalist  system 
as  regards  the  origin  of  living  things.  In  his  Genera/ 
History  ami  Theory  of  flic  Heavens  [translated  in  Kant's 
Cosmogony]  Kant  made  a  very  happy  effort  to  deal  with  the 
structure  and  mechanical  origin  o\  the  universe  on  Newton's 
principles — in  other  words,  to  explain  it  on  mechanical  and 
monistic  principles;  and  this  effort  to  explain  the  origin  o( 
the  universe  by  natural,  efficient  causes  is  still  the  basis  of 
cosmogony.  But  Kant  affirmed  that  this  "principle  o\ 
natural  mechanicism,  without  which  there  can  be  no  real 
science,"  was  quite  incapable  o(  furnishing  an  explanation  o( 
organic  phenomena,  and  especially  of  the  origin  o(  living 
things;  and  that  we  must  turn  to  supernatural  or  final  causes 
for  the  explanation  of  the  origin  o\   these  designed  structures. 


THE  OLDER  PHYLOGEXY 


He  even  went  so  far  as  to  say:  "  It  is  quite  certain  that  we 
cannot  even  satisfactorily  understand,  much  less  explain,  the 
nature  of  an  organism  and  its  internal  forces  on  purelv 
mechanical  principles  ;  it  is  so  certain,  indeed,  that  we  may 
confidently  say:  '  It  is  absurd  for  a  man  to  imagine  even  that 
some  dav  a  Newton  will  arise  who  will  explain  the  origin  of 
a  single  blade  of  grass  by  natural  laws  not  controlled  by 
design' — such  a  hope  is  entirely  forbidden  us."  In  these 
words  Kant  definitely  adopts  the  dualistic  and  teleological 
point  of  view  for  biological  science.1 

Nevertheless,  Kant  deserted  this  point  of  view  at  times, 
particularly  in  several  remarkable  passages  which  I  have 
dealt  with  at  length  in  my  Natural  History  of  Creation 
(chap,  v.),  where  he  expresses  himself  in  the  opposite,  or 
monistic,  sense.  In  fact,  these  passages  would  justify  one, 
as  I  showed,  in  claiming  his  support  for  the  theory  of  evolu- 
tion. Several  very  significant  passages  which  Fritz  Schultze 
has  brought  to  light  in  his  interesting  work,  Kant  unit 
Darwin,  seem  to  give  Kant  the  character  of  being  the  first 
Darwinian  prophet.  He  quite  clearly  enunciates  the  great 
idea  of  an  all-embracing  and  monistic  evolution.  He  speaks 
of  "a  falling  away  from  the  primitive  type  of  the  genus  by 
natural  variations."  In  fact,  he  affirms  that  "  man  originally 
walked  on  four  legs,  and  only  gradually  developed  the  erect 
attitude,  and  raised  himself  so  proudly  above  his  former 
animal  comrades."  However,  these  monistic  passages  are 
only  stray  gleams  of  light;  as  a  rule,  Kant  adheres  in  biology 
to  the  obscure  dualistic  ideas,  according  to  which  the  forces 
at  work  in  inorganic  nature  are  quite  different  from  those  of 
the  organic  world.  This  dualistic  system  prevails  in  academic 
philosophy  to-day — most  of  our  philosophers  still  regarding 
these  two  provinces  as  totally  distinct.  They  put,  on  the  one 
side,  the  inorganic  or  "lifeless"  world,  in  which  there  are  at 
work  only  mechanical  laws,  acting  necessarily  and  without 
design;  and,  on  the  other,  the  province  of  organic  nature,  in 
which    none   of  the   phenomena  can  be  properly  understood, 

1  Kritik  tier  teleologischen  Urtheilskraft,  %%  74  and  79.  [I  translate  Haeckel's 
quotation. — Trans.] 


THE  OLDER  PHYLOGENY  67 


either  as  regards  their  inner  nature  or  their  origin,  except 
in  the  light  of  preconceived  design,  carried  out  by  final  or 
purposive  causes. 

The  prevalence  of  this  unfortunate  dualistic  prejudice 
prevented  the  problem  o\  the  origin  o(  species,  and  the  con- 
nected question  of  the  origin  of  man,  from  being  regarded 
by  the  bulk  of  people  as  a  scientific  question  at  all  until  [859. 
Nevertheless,  a  few  distinguished  students,  free  from  the 
current  prejudice,  be^an,  at  the  commencement  Of  the  nine- 
teenth century,  to  make  a  serious  attack  on  the  problem.  The 
merit  of  this  attaches  particularly  to  what  is  known  as  "  the 
older  school  of  natural  philosophy,"  which  has  been  so  much 
misrepresented,  and  which  included  Jean  Lamarck,  Buffon, 
Geoffroy  St.  Hilaire,  and  Blainville  in  France;  Wolfgang 
Goethe,  Reinhold  Treviranus,  Schelling,  and  Lorentz  Oken 
in  Germany  [and  Erasmus  Darwin  in  England]. 

The  gifted  natural  philosopher  who  treated  this  difficult 
question  with  the  greatest  sagacity  and  comprehensiveness 
was  Jean  Lamarck.  He  was  born  at  Bazentin,  in  Picardv,  on 
August  1st,  1744;  he  was  the  son  of  a  clergyman,  and  was 
destined  for  the  Church.  But  he  turned  to  seek  glory  in  the 
army.  In  his  sixteenth  year  he  distinguished  dimself  by  his 
bravery  in  the  battle  of  Lippstadt,  and  was  then  in  garrison 
in  the  south  o\  France  for  several  years.  Here  he  be^an  to 
study  the  interesting  flora  of  the  Mediterranean  coast,  and  it 
inspired  him  with  a  love  of  botany.  He  resigned  his  com- 
mission, and  in  1 778  published  his  important  work,  Flore 
Francaisc.  for  a  long  time  he  tailed  to  secure  a  place  in 
science,  and  it  was  not  until  his  fiftieth  year  (  1 7<)4)  that  he 
was  offered  the  chair  of  zoology  at  the  museum  o\  the  Jardin 
des  Plantes  at  Paris.  He  then  went  deeper  into  zoology, 
and  he  soon  rendered  as  great  a  service  in  zoological  classifi- 
cation as  he  had  done  in  botany.  In  iSoj  he  published  his 
Considerations  sur  les  corps  vivants,  in  which  we  find  the 
i^erms  o\  his  theory  o\  evolution.  In  1809  appeared  his 
chief  work,  the  famous  Philosophic  Zoohgiqtte,  in  which  he 
developed  his  theory.  In  [815  he  published  his  comprehen- 
sive natural  history  o\    the  vertebrates  (  llistoirc  naturelle  des 


THE  OLDER  PHYLOGEXY 


animaux  sans  vertebres),  in  the  introduction  to  which  his 
theory  is  again  touched  upon.  About  this  time  he  became 
totally  blind.  Fortune,  in  her  jealousy,  never  favoured  him. 
While  his  fortunate  rival,  Cuvier,  rose  to  the  highest  point  of 
scientific  fame  and  prestige  at  Paris,  the  great  Lamarck — far 
greater  than  Cuvier  in  the  vastness  of  his  speculations  and 
his  conception  of  Nature — had  to  struggle  in  solitude  for  the 
necessities  of  life.  His  laborious  life  ended,  in  circumstances 
of  great  poverty,  in  1829. 

Lamarck's  PhUosophie  Zoologiquc1  was  the  first  scientific 
attempt  to  sketch  the  real  course  of  the  origin  of  species,  the 
first  "  natural  history  of  creation  "  of  plants,  animals,  and 
men.  But,  as  in  the  case  of  Wolff's  book,  this  remarkably 
able  work  had  no  influence  whatever  ;  neither  one  nor  the 
other  could  obtain  any  recognition  from  their  prejudiced  con- 
temporaries. No  man  of  science  was  stimulated  to  take  an 
interest  in  the  work,  and  to  develop  the  germs  it  contained  of 
the  most  important  biological  truths.  The  most  distinguished 
botanists  and  zoologists  entirely  rejected  it,  and  did  not  even 
deign  to  reply  to  it.  Cuvier,  who  lived  and  worked  in  the 
same  city,  has  not  thought  lit  to  devote  a  single  syllable  to 
this  great  achievement  in  his  memoir  on  progress  in  the 
sciences,  in  which  the  pettiest  observations  found  a  place.  In 
short,  Lamarck's  Phi/osofihie  Zoologique  shared  the  fate  of 
Wolff's  theory  of  development,  and  was  for  half  a  century 
ignored  and  neglected.  The  German  scientists,  especially 
Oken  and  Goethe,  who  were  occupied  with  similar  specula- 
tions at  the  same  time,  seem  ft)  have  known  nothing  about 
Lamarck's  work.  If  they  had  known  it,  they  would  have 
been  greatly  helped  by  it,  and  might  have  carried  the  theory 
of  evolution  much  farther  than  they  found  it  possible  to  do. 

To  give  an  idea  of  the  great  importance  of  the  Philosophic 
Zoologique,  I  will  briefly  explain  Lamarck's  leading  thought. 
He  held  that  there  was  no  essential  difference  between  living 
and  lifeless  beings.  Nature  is  one  united  and  connected 
system    of  phenomena ;    and    the   forces   which   fashion    the 

1  New  edition,  with  biographical  introduction  by  Charles  Martin.  (Paris, 
'873O 


r/lK  OLDER  PHYLOGENY  (»> 


lifeless  bodies  arc  the  only  ones  at  work  in  the  kingdom  of 
living  things.  We  have,  therefore)  to  use  the  same  method 
of  investigation  and  explanation  in  both  provinces.  Life  is 
only  a  physical  phenomenon.  All  the  plants  and  animals, 
with  man  at  their  head,  are  to  be  explained,  in  structure  and 
life,  by  mechanical  or  efficient  causes,  without  any  appeal 
to  final  causes,  just  as  in  the  case  of  minerals  and  other 
inorganic  bodies.  This  applies  equally  to  the  origin  of  the 
various  species.  We  must  not  assume  any  original  creation, 
or  repeated  creations  (as  in  Cuvier's  theory),  to  explain  this, 
but  a  natural,  continuous,  and  necessary  evolution.  The 
whole  evolutionary  process  has  been  uninterrupted.  All  the 
different  kinds  of  animals  and  plants  which  we  see  to-day,  or 
that  have  ever  lived,  have  descended  in  a  natural  way  from 
earlier  and  different  species  ;  all  come  from  one  common 
stock,  or  from  a  few  common  ancestors.  These  remote 
ancestors  must  have  been  quite  simple  organisms  of  the 
lowest  type,  arising  by  spontaneous  generation  from 
inorganic  matter.  The  succeeding  species  have  been 
constantly  modified  by  adaptation  to  their  varying  environ- 
ment (especially  by  use  and  habit),  and  have  transmitted 
their  modifications  to  their  successors  by  heredity. 

These  are  the  chief  outlines  of  Lamarck's  theory,  which 
we  now  call  the  theory  of  descent  or  "  transformism,"  and 
which  was  unrecognised  till  Darwin  took  it  up  and  gave  it 
fresh  support  fifty  years  later.  Lamarck  is  the  real  founder  of 
the  theory  of  evolution,  and  it  is  incorrect  to  speak  of  Darwin 
as  its  first  champion.  Lamarck  was  the  first  to  formulate  as 
a  scientific  theory  the  natural  origin  of  living  things,  includ- 
ing man,  and  to  push  the  theory  to  its  extreme  conclusions 
the  rise  of  the  earliest  organisms  by  spontaneous  generation 
(or  abiogenesis)  and  the  descent  of  man  from  the  nearest 
related  mammal,  the  ape. 

Lamarck  sought  to  explain  this  last  point,  which  is  ol 
especial  interest  to  us  here,  by  the  same  agencies  which  he 
found  at  work  in  the  natural  origin  of  the  plant  and  animal 
species.  He  considered  use  and  habit  (adaptation)  on  the 
One  hand,  and  heredity  on   the  other,  to  be  the  chief  of  these 


THE  OLDER  PHYLOGEXY 


agencies.  The  most  important  modifications  of  the  organs  of 
plants  and  animals  are  due,  in  his  opinion,  to  the  function  of 
these  very  organs,  or  to  the  use  or  disuse  of  them.  To  give 
a  few  examples,  the  woodpecker  and  the  humming-bird  have 
got  their  peculiarly  long  tongues  from  the  habit  of  extracting 
their  food  with  their  tongues  from  deep  and  narrow  folds  or 
canals  ;  the  frog  has  developed  the  web  between  his  toes  by 
his  own  swimming  ;  the  giraffe  has  lengthened  his  neck  by 
stretching  up  to  the  higher  branches  of  trees,  and  so  on.  It 
is  quite  certain  that  this  use  or  disuse  of  organs  is  a  most 
important  factor  in  organic  development,  but  it  is  not 
sufficient  to  explain  the  origin  of  species. 

To  adaptation  we  must  add  heredity  as  the  second  and 
not  less  important  agency,  as  Lamarck  perfectly  recognised. 
He  said  that  the  modification  of  the  organs  in  any  one 
individual  by  use  or  disuse  was  slight,  but  that  it  was 
increased  by  accumulation  in  passing  by  heredity  from 
generation  to  generation.  But  he  missed  altogether  the 
principle  which  Darwin  afterwards  found  to  be  the  chief 
factor  in  the  theory  of  transformation — namely,  the  principle 
of  natural  selection  in  the  struggle  for  existence.  It  was 
partly  owing  to  his  failure  to  detect  this  supremely  important 
element,  and  partly  to  the  poor  condition  of  all  biological 
science  at  the  time,  that  Lamarck  did  not  succeed  in  establish- 
ing more  firmly  his  theory  of  the  common  descent  of  man 
and  the  other  animals. 

Lamarck  tried  to  explain  the  descent  of  man  from  the  ape 
chiefly  by  advance  in  the  habits  of  the  ape,  and  by  a  pro- 
gressive development  and  use  of  its  organs  and  the  trans- 
mission to  posterity  of  the  modifications  thus  produced.  He 
considered  the  most  important  of  these  improvements  to  be 
man's  erect  attitude,  the  modification  of  the  hands  and  feet, 
and  the  acquisition  of  speech  and  accompanying  develop- 
ment of  the  brain.  He  believed  that  the  man-like  apes, 
which  were  man's  ancestors,  had  taken  the  first  step  towards 
humanity  when  they  ceased  to  climb  trees  and  began  to  walk 
erect.  This  led  to  the  distinctive  human  carriage,  the  modifi- 
cation   of    the    vertebral    column    and   the    pelvis,    and    the 


THE  Ol.ni-.K  PHYLOGENY 


differentiation  of  the  upper  and  lower  limbs  :  the  upper  limbs 
became  hands,  and  were  used  for  grasping  and  touching 
things,  while  the  lower  were  confined  to  locomotive  purposes, 
and  became  feet  pure  and  simple. 

As  a  result  of  this  complete  change  of  habits,  and  in 
virtue  of  the  correlation  of  the  various  organs  and  their 
functions,  a  number  of  other  modifications  were  caused. 
Thus  the  change  in  diet  led  to  a  modification  of  the  jaws  and 
teeth,  and  therefore  of  the  whole  face.  The  tail  was  no  longer 
of  any  use,  and  it  gradually  disappeared.  And  as  these  apes 
lived  in  troops  and  had  regular  family  relations  (as  is  the 
case  to-dav  with  the  higher  apes),  the  gregarious  or  social 
instincts  were  strongly  developed.  The  simple  sound-speech 
of  the  ape  grew  into  the  articulate  speech  of  the  man  ;  abstract 
ideas  were  formed  from  the  groups  of  concrete  impressions. 
Thus  step  by  step  the  brain  advanced,  and  with  it  the 
larynx — the  organ  of  mind  simultaneously  with  the  organ  of 
speech.  In  these  most  interesting  speculations  of  Lamarck 
we  have  the  germs  of  a  sound  theory  of  the  evolution  of  man. 
(Cf.   Packard). 

Independentlv  of  Lamarck,  the  older  German  school  of 
natural  philosophv,  especially  Reinhold  Treviranus,  in  his 
Biologic  (1802),  and  Lorentz  Oken,  in  his  NaturphUosophie 
(1809),  turned  its  attention  to  the  problem  of  evolution  about 
the  end  of  the  eighteenth  and  beginning  of  the  nineteenth 
century.  I  have  described  its  work  in  my  Natural  History  of 
Creation  (chap.  iv.).  Here  I  can  only  deal  with  the  brilliant 
genius  whose  evolutionary  ideas  are  of  special  interest — the 
greatest  of  German  poets,  Wolfgang  Goethe.  With  his  keen 
eye  for  the  beauties  of  nature,  and  his  profound  insight  into 
its  life,  Goethe  was  early  attracted  to  the  study  of  various 
natural  sciences.  It  was  the  favourite  occupation  of  his 
leisure  hours  throughout  life.  He  gave  particular  and 
protracted  attention  to  the  theory  of  colours.  But  the  most 
valuable  of  his  scientific  studies  are  those  which  relate  to  that 
"living,  glorious,  precious  thing,"  the  organism.  lie  made 
profound  research  into  the  science  of  structures  or  morpho- 
logy (morphae        forms).      Here,  with  the  aid  of  comparative 


THE  OLDER  PHYLOGEXY 


anatomy,  he  obtained  the  most  brilliant  results,  and  went  far 
in  advance  of  his  time.  I  may  mention,  in  particular,  his 
vertebral  theory  of  the  skull,  his  discovery  of  the  pineal  gland 
in  man,  his  system  of  the  metamorphosis  of  plants,  etc. 
These  morphological  studies  led  Goethe  on  to  research  into 
the  formation  and  modification  of  organic  structures  which 
we  must  count  as  the  first  germ  of  the  science  of  evolution. 
He  approaches  so  near  to  the  theory  of  descent  that  we 
must  regard  him,  after  Lamarck,  as  one  of  its  earliest 
founders.  It  is  true  that  he  never  formulated  a  complete 
scientific  theory  of  evolution,  but  we  find  a  number  of 
remarkable  suggestions  of  it  in  his  splendid  miscellaneous 
essays  on  morphology.  Some  of  them  are  really  among 
the  very  basic  ideas  of  the  science  of  evolution.  I  will 
quote  here  only  one  or  two  of  the  most  remarkable  passages  : 
"  We  have  got  far  enough,  then,  to  say  confidently  that  all 
the  higher  organic  natures,  in  which  we  include  the  fishes, 
amphibia,  birds,  and  mammals,  with  man  at  their  head,  are 
made  after  one  primitive  type,  and  this  only  oscillates  a  little 
to  one  side  or  other  of  its  steady  features,  and  daily  advances 
and  is  modified  by  reproduction"  (1796).  This  "primitive 
type,"  on  which  even  man  is  modelled,  corresponds  to  our 
common  ancestral  form  of  the  vertebrate  stem,  from  which  all 
the  different  species  of  vertebrates  have  arisen  by  "  incessant 
formation,  modification,  and  reproduction."  In  another 
place  Goethe  says  (1807)  :  "When  we  compare  plants  and 
animals  in  their  most  rudimentary  forms,  it  is  almost 
impossible  to  distinguish  between  them.  But  we  may  say 
that  the  plants  and  animals,  beginning  with  an  almost 
inseparable  closeness,  gradually  advance  along  two  divergent 
lines,  until  the  plant  at  last  grows  in  the  solid,  enduring  tree 
and  the  animal  attains  in  man  to  the  highest  degree  of 
mobility  and  freedom." 

That  Goethe  was  not  merely  speaking  in  a  poetical,  but  in 
a  literal  genealogical,  sense  of  this  close  affinity  of  organic 
forms  is  clear  from  other  remarkable  passages  in  which  he 
treats  of  their  variety  in  outward  form  and  unity  in  internal 
structure.     He  believes  that  every  living  thing  has  arisen  by 


THE  OLDER  PHYLOGENY 


the  interaction  of  two  opposing  formative  forces  or  impulses. 
The  internal  or  "centripetal  "  force,  the  type  or  "  impulse  to 
specification,"  seeks  to  maintain  the  constancy  of  the  specific 
forms  in  the  succession  of  generations:  this  is  heredity.  The 
external  or  "  centrifugal  "  force,  the  element  of  variation  or 
"  impulse  to  metamorphosis,"  is  continually  modifying  the 
specie^  by  changing  their  environment  :  this  is  adaptation. 
In  these  significant  conceptions  Goethe  approaches  very 
close  to  a  recognition  of  the  two  great  mechanical  factors 
which  we  now  assign  as  the  chief  causes  of  the  formation  of 
species. 

However,  in  order  to  appreciate  Goethe's  views  on 
morphology,  one  must  associate  his  decidedly  monistic 
conception  of  nature  with  his  pantheistic  philosophy.  The 
warm  and  keen  interest  with  which  he  followed,  in  his  last 
years,  the  controversies  of  contemporary  French  scientists, 
and  especially  the  struggle  between  Cuvi^er  and  Geoffrey  St. 
Ililaire  (see  chap.  iv.  of  The  Natural  History  of  Creation), 
is  very  characteristic.  It  is  also  necessary  to  be  familiar  with 
his  style  and  general  tenour  of  thought  in  order  to  appreciate 
rightly  the  many  allusions  to  evolution  found  in  his  writings. 
Otherw  ise,  one  is  apt  to  make  serious  errors. 

In  a  lecture  that  I  delivered  in  [882  at  the  Congress  of 
German  scientists  and  medical  men  at  Eisenach  I  made  a 
rather  full  comparison  of  the  scientific  ideas  of  Darwin, 
Goethe,  and  Lamarck,  and  showed  their  important  bearing 
on  the  pantheistic  philosophv.  In  my  opinion,  these  three 
greatest  figures  in  modern  thought  stand  on  the  common 
ground  of  Monism,  or  the  system  which  teaches  the  unity  of 
the  universe  on  scientific  grounds.  All  held  the  belief  in  the 
unity  of  God  and  Xature  which  was  defended  by  Giordano 
Bruno  and  Spinoza,  and  which  Goethe  expressed  so  nobly  in 
his  writings  on  God  and  the  World.  We  can  understand, 
therefore,  the  lively  interest  which  Goethe  maintained  till  his 
last  days  in  the  highest  questions  of  biology.  The  passages 
which  I  have  quoted  on  the  title-pages  of  the  chapters  in  my 
GenereUe  Morphologie  show  how  firm  a  grasp  he  had  ot  the 
intimate  genetic  relation  of  all  organic  forms.     Me  approached 


74  THE  OLDER  PHYLOGEXY 

so  close,  at  the  end  of  the  eighteenth  century,  to  the  principles 
of  the  science  of  evolution  that  he  may  well  be  described  as 
the  first  forerunner  of  Darwin,  although  he  did  not  go  so  far 
as  to  formulate  evolution  as  a  scientific  system,  as  Lamarck 
did. 


THE  MODERN  SCIENCE  OF  EVOLUTION 


its  influence,  new  structures,  or  alterations  of  structure,  are 

produced  ;  and  these  are  purposive  in  the  sense  that  they 
serve  the  organism  when  formed,  but  they  were  produced 
without  any  pre-eonceived  aim. 

This  simple  idea  is  the  central  thought  of  Darwinism,  or 
the  theory  of  selection.  Darwin  conceived  this  idea  at  an 
early  date,  and  then,  for  more  than  twenty  years,  worked  at 
the  collection  of  empirical  evidence  in  support  of  it  before  he 
published  his  theory.  I  have  described  the  chief  features  of 
his  method,  his  life,  and  his  writings  in  my  Natural  History 
of  Creation.  The  ample  biography,  in  three  volumes,  pub- 
lished by  his  son,  Francis  Darwin,  in  1S87,  gives  full 
information  about  him.  Here  1  will  only  refer  to  some  of  the 
salient  points.  Charles  Darwin  was  born  on  February  12th, 
1S09,  at  Shrewsbury,  where  his  father,  Robert  Darwin,  had  a 
medical  practice.  Mis  grandfather,  Erasmus  Darwin,  was  an 
able  scientist  of  the  older  school  of  natural  philosophv,  who 
published  a  number  of  natural-philosophic  works  about  the 
end  of  the  eighteenth  century.  The  most  important  of  them 
is  his  Zoonomta,  published  in  1704,  in  which  he  expounds 
views  similar  to  those  of  Goethe  and  Lamarck,  without,  how- 
ever, knowing  anything  of  the  work  of  these  contemporaries. 
By  the  law  of  latent  heredity,  or  "atavism,"  Erasmus 
Darwin  transmitted  a  part  of  his  ability  to  his  grandson 
Charles,  though  no  trace  of  it  is  found  in  his  son  Robert. 
This  is  a  very  interesting  case  of  atavism,  a  process  which 
Charles  Darwin  himself  treated  so  admirably.  However,  in 
the  writings  of  the  grandfather  the  plastic  imagination  rather 
outran  the  judgment,  while  in  Charles  Darwin  the  two  were 
better  balanced.  As  many  narrow-minded  scientists  of  our 
own  day  regard  the  imagination  as  superfluous  in  biology, 
and  think  their  lack  of  it  a  great  advantage  in  the  way  of 
"  exactness,"  it  is  interesting  to  call  attention  to  a  striking 
saying  of  a  gifted  man  of  science  who  was  himself  one  of  the 
founders  of  the  "exact"  or  strictly  empirical  school.  Johannes 
Miiller,  the  German  Cuvier,  whose  works  will  ever  remain  a 
model  of  accurate  research,  declared  that  a  constant  inter- 
action   and   harmonious  adjustment  of  the    imagination   and 


78  THE  MODERX  SCIEXCE  OF  EVOLUTIOX 

the  intellect  was  an  indispensable  condition  for  making  great 
discoveries. 

Charles  Darwin  was  fortunate  enough  to  take  part  in  a 
scientific  expedition  at  the  close  of  his  university  career  in  his 
twenty-second  year.  This  lasted  five  years,  and  greatly 
stimulated  him  and  enriched  his  fund  of  knowledge.  At  the 
very  beginning  of  it,  as  soon  as  he  landed  in  America,  he 
was  attracted  by  a  number  of  phenomena  which  suggested 
the  chief  problem  of  his  life — the  question  of  the  origin  of 
species.  The  instructive  facts  of  the  geographical  distribu- 
tion of  species,  on  the  one  hand,  and  the  relation  of  living  to 
dead  species  of  the  same  locality  on  the  other,  prompted  him 
to  surmise  that  closely-related  species  must  have  descended 
from  a  common  stem  form.  Then,  at  the  close  of  his  voyage, 
when  he  devoted  himself  for  a  year  with  great  vigour  to  the 
systematic  study  of  domestic  animals  and  garden  plants,  he 
noticed  the  obvious  analogies  in  structures  between  them  and 
the  corresponding  species  in  the  wild  state.  But  he  did  not 
come  to  conceive  the  chief  point  of  his  theory,  natural 
selection  through  the  struggle  for  life,  until  he  read  Malthus's 
famous  Essay  on  Population.  He  then  saw  clearly  the 
analogy  between  the  relations  of  population  and  over-popu- 
lation in  civilised  communities  and  the  mutual  relations  of 
animals  and  plants  in  a  natural  state.  For  many  years  he 
collected  material  to  give  a  massive  support  to  his  theory. 
At  the  same  time,  he  made  a  number  of  experiments  himself 
in  artificial  selection,  and  gave  special  attention  to  the  action 
of  selection  on  tame  pigeons.  The  quietness  of  his  life  on 
his  estate  at  Down,  near  Beckenham,  gave  him  requisite 
leisure.  He  died  there  on  April  19th,  18S2,  working  assidu- 
ously until  death  at  the  establishment  of  his  epoch-making 
theory  by  new  discoveries. 

Darwin  did  not  publish  any  account  of  his  theory  until 
1858,  when  Alfred  Russel  Wallace,  who  had  independently 
reached  the  same  theory  of  selection,  published  his  own  work. 
In  the  following  year  appeared  the  Origin  of  Species,  in  which 
he  developes  it  at  length  and  supports  it  with  a  mass  of  proof. 
As    I    have    given    my  opinion   on   it  fully  in   my   Generelle 


THE  MODERN  SCIENCE  OF  EVOLUTION  79 

Morphologic  and  Natural  History  of  Creation,  I  need  not  stay 

to  do  so  here,  and  will  only  add  a  word  on  the  essence  oi  the 
Darwinian  theory,  on  the  understanding  of  which  all  the  rest 
depends.  This  is  the  simple  prineiple  that  the  stru^le  for 
lite  modifies  living  things  in  the  natural  condition,  and  pro- 
duces  new  species,  through  the  same  agencies  which  man 
employs  in  artificially  forming  new  varieties  of  animals  and 
plants.  These  agencies  virtually  exercise  a  selection  among 
the  individuals  brought  into  existence,  heredity  and  adapta- 
tion acting  together  throughout  as  the  chief  plastic  forces." 

Darwin's  younger  contemporary,  Alfred  Russel  Wallace, 
the  famous  traveller,  had  reached  the  same  conclusion.  But 
he  had  not  so  clear  a  perception  as  Darwin  of  the  effectiveness 
of  natural  selection  in  forming  species,  and  did  not  develop 
the  theory  so  fully.  Nevertheless,  Wallace's  writings,  espe- 
cially those  on  mimicry,  etc.,  and  an  admirable  work  on 
The  Geographical  Distribution  of  Animals,  contain  many  fine 
original  contributions  to  the  theory  of  selection.  Unfortu- 
nately, this  gifted  scientist  has  since  devoted  himself  to 
spiritism. 

Darwin's  Origin  of  Species  had  an  extraordinary  influence, 
though  not  at  first  on  the  experts  of  the  science.  It  took 
zoologists  and  botanists  several  years  to  recover  from  the 
astonishment  into  which  they  had  been  thrown  through  the 
revolutionary  idea  of  the  work.  But  its  influence  on  the 
special  sciences  with  which  we  zoologists  and  botanists  arc 
concerned  has  increased  from  year  to  year;  it  has  introduced 
a  most  healthy  fermentation  in  every  branch  of  biology, 
especially  in  comparative  anatomy  and  ontogeny,  and  in 
zoological  and  botanical  classification.  In  this  way  it  has 
brought  about  almost  a  revolution  in  the  prevailing  views. 

However,  the  point  which  chiefly  concerns  us  here — the 
extension  of  the  theory  to  man — was  not  touched  at  all  in 
Darwin's  first  work  in  1859.  It  was  believed  for  several  years 
that    he   had    no   thought  of  applying    his  principles  to  man, 

'  Darwin  and  Wallace  arrived  al  the  theory  quite  independently.  Vide 
Wallace's  Contributions  In  the  Theory  of  Natural  Selection  ( [870)  and  Darwinism 

(■891). 


THE  MODERX  SCIENCE  OF  EVOLVTIOX 


but  that  he  shared  the  current  idea  of  man  holding  a  special 
position  in  the  universe.  Not  only  ignorant  laymen  (espe- 
cially several  theologians),  but  also  a  number  of  men  of 
science,  said  very  naively  that  Darwinism  in  itself  was  not  to  be 
opposed  ;  that  it  was  quite  right  to  use  it  to  explain  the  origin 
of  the  various  species  of  plants  and  animals,  but  that  it  was 
totally  inapplicable  to  man. 

In  the  meantime,  however,  it  seemed  to  a  good  many 
thoughtful  people,  laymen  as  well  as  scientists,  that  this  was 
wrong;  that  the  descent  of  man  from  some  other  animal 
species,  and  immediately  from  some  ape-like  mammal, 
followed  logically  and  necessarily  from  Darwin's  reformed 
theory  of  evolution.  Many  of  the  acuter  opponents  of  the 
theory  saw  at  once  the  justice  of  this  position,  and,  as  this 
consequence  was  intolerable,  they  wanted  to  get  rid  of  the 
whole  theory. 

The  first  scientific  application  of  the  Darwinian  theory  to 
man  was  made  by  Huxley,  the  greatest  zoologist  in  England. 
This  able  and  learned  scientist,  to  whom  zoology  owes  much 
of  its  progress,  published  in  1S63  a  small  work  entitled 
Evidence  as  to  Man's  Place  in  Mature.  In  the  extremely 
important  and  interesting  lectures  which  made  up  this  work 
he  proved  clearly  that  the  descent  of  man  from  the  ape 
followed  necessarily  from  the  theory  of  descent.  If  that 
theory  is  true,  we  are  bound  to  conceive  the  animals  which 
most  closely  resemble  man  as  those  from  which  humanity 
has  been  gradually  evolved.  About  the  same  time  Carl 
Vogt  published  a  larger  work  on  the  same  subject — Vorle- 
sungen  iiber  den  menschen  seine  Stellung  in  der  Schcpjung 
and  in  der  Geschichte  der  Erde.  We  must  also  mention 
Gustav  Jaeger  and  Friedrich  Rolle  among  the  zoologists  who 
accepted  and  taught  the  theory  of  evolution  immediately 
after  the  publication  of  Darwin's  book,  and  maintained  that 
the  descent  of  man  from  the  lower  animals  logically  followed 
from  it.  The  latter  published,  in  1866,  a  work  on  the  origin 
and  position  of  man. 

About  the  same  time  I  attempted,  in  the  second  volume  of 
my  Generelle  Morphologie  der  Organismen  (1866),  to  apply  the 


THE  MODERN  SCIENCE  OF  EVOLUTION 


theory  of  evolution  to  the  whole  organic  kingdom,  including 
man.1  I  endeavoured  to  sketch  the  probable  ancestral  trees 
of  the  various  classes  of  the  animal  world,  the  protists,  and 
the  plants,  as  it  seemed  necessary  to  do  on  Darwinian 
principles,  and  as  we  can  actually  do  now  with  a  high  degree 
of  confidence.  If  the  theory  of  descent  which  Lamarck  first 
clearly  formulated  and  Darwin  thoroughly  established  is  true, 
we  seem  to  be  able  to  draw  up  a  natural  classification  of 
plants  and  animals  in  the  light  of  their  genealogy,  and  to 
conceive  the  large  and  small  divisions  of  the  system  as  the 
branches  and  twigs  of  an  ancestral  tree.  The  eight  genealo- 
gical tables  which  I  inserted  in  the  second  volume  of  the 
Generelle  Morphologie  are  the  first  sketches  of  their  kind.  In 
the  twenty-seventh  chapter,  particularly,  I  trace  the  chief 
stages  in  man's  ancestry,  as  far  as  it  is  possible  to  follow  it 
through  the  vertebrate  stem.  I  tried  especially  to  determine, 
as  well  as  one  could  at  that  time,  the  position  of  man  in  the 
classification  of  the  mammals  and  its  genealogical  signifi- 
cance. I  have  greatly  improved  this  attempt,  and  treated  it 
in  a  more  popular  form,  in  chaps,  xxvi.-xxviii.  of  ray  Natural 
History  of  Creation  (1868).2 

It  was  not  until  1871,  twelve  years  after  the  appearance  of 
The  Origin  of  Species,  that  Darwin  published  the  famous 
work  which  made  the  much-contested  application  of  his  theory 
to  man,  and  crowned  the  splendid  structure  of  his  system. 
This  important  work  was  The  Descent  of  Man,  and  Selection  in 
Relation  to  Sex.  In  this  Darwin  expressly  drew  the  conclu- 
sion, with  rigorous  logic,  that  man  also  must  have  been 
developed  out  of  lower  species,  and  described  the  important 
part  played  by  sexual  selection  in  the  elevation  of  man  and 
the  other  higher  animals.  He  showed  that  the  careful 
selection  which  the  sexes  exercise  on  each  other  in  regard  to 
sexual  relations  and  procreation,  and  the  aesthetic  feeling 
which  the  higher  animals  develop  through  this,  are  of  the 

1  Huxley   spoke   of  this   as   "one   of    the   greatest   scientific   works  ever 
published."    -Tkans. 

*  Of  which  Darwin  said  that  the  Descent  of  Man  would  probably  never  have 
been  written  if  he  had  seen  it  earlier. — TRANS. 

G 


THE  MODERN  SCIENCE  OF  EVOLUTION 


utmost  importance  in  the  progressive  development  of  forms 
and  the  differentiation  of  the  sexes.  The  males  choosing  the 
handsomest  females  in  one  class  of  animals,  and  the  females 
choosing  only  the  finest-looking  males  in  another,  the  special 
features  and  the  sexual  characteristics  are  increasingly 
accentuated.  In  fact,  some  of  the  higher  animals  develop  in 
this  connection  a  finer  taste  and  less  prejudiced  judgment 
than  man  himself.  But,  even  as  regards  man,  it  is  to  this 
sexual  selection  that  we  owe  the  family-life,  which  is  the 
chief  foundation  of  civilisation.  The  rise  of  the  human  race 
is  due  for  the  most  part  to  the  advanced  sexual  selection 
which  our  ancestors  exercised  in  choosing  their  mates.  (Cf. 
the  eleventh  chapter  of  the  Natural  History  of  Creation  and 
the  second  volume  of  the  Generelle  Morphologic.) 

Darwin  accepted  in  the  main  the  general  outlines  of 
man's  ancestral  tree,  as  I  gave  it  in  the  Generelle  Morpho- 
logie  and  the  Natural  History  of  Creation,  and  admitted  that 
his  studies  led  him  to  the  same  conclusion.  That  he  did  not 
at  once  apply  the  theory  to  man  in  his  first  work  was  a 
commendable  piece  of  discretion  ;  such  a  sequel  was  bound 
to  excite  the  strongest  opposition  to  the  whole  theory.  The 
first  thing  to  do  was  to  establish  it  as  regards  the  animal 
and  plant  worlds.  The  subsequent  extension  to  man  was 
bound  to  be  made  sooner  or  later. 

It  is  important  to  understand  this  very  clearly.  If  all 
living  things  come  from  a  common  root,  man  must  be 
included  in  the  general  scheme  of  evolution.  On  the  other 
hand,  if  the  various  species  were  separately  created,  man, 
too,  must  have  been  created,  and  not  evolved.  We  have  to 
choose  between  these  two  alternatives.  This  cannot  be  too 
frequently  or  too  strongly  emphasised.  Either  all  the 
species  of  animals  and  plants  are  of  supernatural  origin — 
created,  not  evolved — and  in  that  case  man  also  is  the 
outcome  of  a  creative  act,  as  religion  teaches;  or  the  different 
species  have  been  evolved  from  a  few  common,  simple 
ancestral  forms,  and  in  that  case  man  is  the  highest  fruit 
of  the  tree  of  evolution. 

We  may  state  this  briefly  in  the  following  principle: — The 


THE  MODERN  SCIENCE  OF  EVOLUTION  83 

descent  of  man  from  the  lower  animals  is  a  special  deduction 
which  inevitably  follows  from  the  general  inductive  lam  of  the 
whole  theory  of  evolution.  In  this  principle  we  have  a  clear 
and  plain  statement  of  the  matter.  Evolution  is  in  reality 
nothing  but  a  great  induction,  which  we  are  compelled  to 
make  by  the  comparative  study  of  the  most  important  facts  of 
morphology  and  physiology.  But  we  must  draw  our  con- 
clusion according  to  the  laws  of  induction,  and  not  attempt 
to  determine  scientific  truths  by  direct  measurement  and 
mathematical  calculation.  In  the  study  of  living  things  we 
can  scarcely  ever  directly  and  fully,  and  with  mathematical 
accuracy,  determine  the  nature  of  phenomena,  as  is  done  in 
the  simpler  study  of  the  inorganic  world — in  chemistry, 
physics,  mineralogy,  and  astronomy.  In  the  latter, especially, 
we  can  always  use  the  simplest  and  absolutely  safest  method 
— that  of  mathematical  determination.  But  in  biology  this 
is  quite  impossible  for  various  reasons ;  one  very  obvious 
reason  being  that  most  o(  the  facts  of  the  science  are  very 
complicated  and  much  too  intricate  to  allow  a  direct  mathe- 
matical analysis.  The  greater  part  of  the  phenomena  that 
biology  deals  with  are  complicated  historical  processes,  which 
are  related  to  a  far-reaching  past,  and  as  a  rule  can  only  be 
approximate!}'  estimated.  Hence  we  have  to  proceed  by 
induction — that  is  to  say,  to  draw  general  conclusions,  stage 
by  Stage,  and  with  proportionate  confidence,  from  the 
accumulation  of  detailed  observations.  These  inductive 
conclusions  cannot  command  absolute  confidence,  like  mathe- 
matical axioms;  but  they  approach  the  truth,  and  gain 
increasing  probability,  in  proportion  as  we  extend  the  basis 
ot  observed  facts  on  which  we  build.  The  importance  of 
these  inductive  laws  is  not  diminished  from  the  circumstance 
that  they  are  looked  upon  merely  as  temporary  acquisitions  of 
science,  and  may  be  improved  to  any  extent  in  the  progress 
of  scientific  knowledge.  The  same  may  be  said  of  the 
attainments  o(  many  other  sciences,  such  as  geology  or 
archeology.  1  lowever  much  they  may  be  altered  and  im- 
proved in  detail  in  the  course  of  time,  these  inductive 
truths  may  retain  their  substance  unchanged. 


84  THE  MODERN  SCIENCE  OF  EVOLUTION 

Now,  when  we  say  that  the  theory  of  evolution  in  the  sense 
of  Lamarck  and  Darwin  is  an  inductive  law — -in  fact,  the 
greatest  of  all  biological  inductions — we  rely,  in  the  first  place, 
on  the  facts  of  paleontology.  This  science  gives  us  some  direct 
acquaintance  with  the  historical  phenomena  of  the  changes  of 
species.  From  the  situations  in  which  we  find  the  fossils  in 
the  various  strata  of  the  earth  we  gather  confidently,  in  the 
first  place,  that  the  living  population  of  the  earth  has  been 
gradually  developed,  as  clearly  as  the  earth's  crust  itself; 
and  that,  in  the  second  place,  several  different  populations 
have  succeeded  each  other  in  the  various  geological  periods. 
Modern  geology  teaches  that  the  formation  of  the  earth  has 
been  gradual,  and  unbroken  by  any  violent  revolutions. 
And  when  we  compare  together  the  various  kinds  of  animals 
and  plants  which  succeed  each  other  in  the  history  of  our 
planet,  we  find,  in  the  first  place,  a  constant  and  gradual 
increase  in  the  number  of  species  from  the  earliest  times  until 
the  present  day;  and,  in  the  second  place,  we  notice  that  the 
forms  in  each  great  group  of  animals  and  plants  also 
constantly  improve  as  the  ages  advance.  Thus,  of  the 
vertebrates  there  are  at  first  only  the  lower  fishes ; 
then  come  the  higher  fishes,  and  later  the  amphibia.  Still 
later  appear  the  three  higher  classes  of  vertebrates — the 
reptiles,  birds,  and  mammals,  for  the  first  time;  only  the 
lowest  and  least  perfect  forms  of  the  mammals  are  found  at 
first;  and  it  is  only  at  a  very  late  period  that  placental 
mammals  appear,  and  man  belongs  to  the  latest  and  youngest 
branch  of  these.  Thus  perfection  of  form  increases  as  well  as 
variety  from  the  earliest  to  the  latest  stage.  That  is  a  fact  of 
the  greatest  importance.  It  can  only  be  explained  by  the 
theory  of  evolution,  with  which  it  is  in  perfect  harmony.  If 
the  different  groups  of  plants  and  animals  do  really  descend 
from  each  other,  we  must  expect  to  find  this  increase  in  their 
number  and  perfection  under  the  influence  of  natural  selection, 
just  as  the  succession  of  fossils  actually  discloses  it  to  us. 

Comparative  anatomy  furnishes  a  second  series  of  facts 
which  are  of  great  importance  for  the  forming  of  our  inductive 
law.       This    branch    of    morphology     compares    the    adult 


THE  MODERN  SCIENCE  OF  EVOLUTION  85 

Structures  of  living  things,  and  seeks  in  the  great  variety  of 
organic  forms  the  stable  and  simple  law  of  organisation,  or 
the  common  type  or  structure.  Since  Cuvier  founded  this 
science  at  the  beginning  of  the  nineteenth  century  it  has  been 
a  favourite  study  of  the  most  distinguished  scientists.  Even 
before  Cuvier's  time  Goethe  had  been  greatly  stimulated  by  it, 
and  induced  to  take  up  the  study  of  morphology.  Compara- 
tive osteology,  or  the  philosophic  study  and  comparison  of  the 
bony  skeleton  of  the  vertebrates — one  of  its  most  interesting 
sections — especially  fascinated  him,  and  led  him  to  form  the 
theory  of  the  skull  which  I  mentioned  before.  Comparative 
anatomy  shows  that  the  internal  structure  of  the  animals  of 
each  stem  and  the  plants  of  each  class  is  the  same  in  its 
essential  features,  however  much  they  differ  in  external 
appearance.  Thus  man  has  so  great  a  resemblance  in  the 
chief  features  of  his  internal  organisation  to  the  other 
mammals  that  no  comparative  anatomist  has  ever  doubted 
that  he  belongs  to  this  class.  The  whole  internal  structure  of 
the  human  body,  the  arrangement  of  his  various  systems  of 
organs,  the  distribution  of  the  bones,  muscles,  blood-vessels, 
etc.,  and  the  whole  structure  of  these  organs  in  the  larger  and 
the  finer  scale,  agree  so  closely  with  those  of  the  other 
mammals  (such  as  the  apes,  rodents,  ungulates,  cetacea, 
marsupials,  etc.)  that  their  external  differences  are  of  no 
account  whatever.  We  learn  further  from  comparative 
anatomy  that  the  chief  features  of  animal  structure  are  so 
similar  in  the  various  classes  (fifty  to  sixty  in  number  alto- 
gether) that  they  may  all  be  comprised  in  from  eight  to 
twelve  great  groups.  But  even  in  these  groups,  the  stem- 
forms  or  animal  types,  certain  organs  (especially  the  alimen- 
tary canal)  can  be  proved  to  have  been  originally  the  same 
for  all.  We  can  only  explain  by  the  theory  of  evolution  this 
essential  unity  in  internal  structure  of  all  these  animal  forms 
that  differ  so  much  in  outward  appearance.  This  wonderful 
fact  can  only  be  really  understood  and  explained  when  \\l- 
regard  the  internal  resemblance  as  an  inheritance  from 
common-stem  forms,  and  the  external  differences  as  the  effect 
of  adaptation  to  different  environments. 


THE  MODERN  SCIENCE  OF  EVOLUTION 


In  recognising  this,  comparative  anatomy  has  itself 
advanced  to  a  higher  stage.  Gegenbaur,  the  most  distin- 
guished of  living  students  of  this  science,  says  that  with  the 
theory  of  evolution  a  new  period  began  in  comparative 
anatomy,  and  that  the  theory  in  turn  found  a  touchstone  in 
the  science.  "  Up  to  now  there  is  no  fact  in  comparative 
anatomy  that  is  inconsistent  with  the  theory  of  evolution  ; 
indeed,  they  all  lead  to  it.  In  this  way  the  theory  receives 
back  from  the  science  all  the  service  it  rendered  to  its 
method."  Until  then  students  had  marvelled  at  the  wonderful 
resemblance  of  living  things  in  their  inner  structure  without 
being  able  to  explain  it.  We  are  now  in  a  position  to  explain 
the  causes  of  this,  by  showing  that  this  remarkable  agree- 
ment is  the  necessary  consequence  of  the  inheriting  of 
common  stem-forms  ;  while  the  striking  difference  in  outward 
appearance  is  a  result  of  adaptation  to  changes  of  environ- 
ment. Heredity  and  adaptation  alone  furnish  the  true 
explanation. 

But  one  special  part  of  comparative  anatomy  is  of  supreme 
interest  and  of  the  utmost  philosophic  importance  in  this 
connection.  This  is  the  science  of  rudimentary  or  useless 
organs  ;  I  have  given  it  the  name  of  "  dysteleology  "  in  view 
of  its  philosophic  consequences.  Nearly  every  organism 
(apart  from  the  very  lowest),  and  especially  every  highly- 
developed  animal  or  plant,  including  man,  has  one  or  more 
organs  which  are  of  no  use  to  the  body  itself,  and  have  no 
share  in  its  functions  or  vital  aims.  Thus  we  all  have,  in 
various  parts  of  our  frame,  muscles  which  we  never  use,  as, 
for  instance,  in  the  shell  of  the  ear  and  adjoining  parts.  In 
most  of  the  mammals,  especially  those  with  pointed  ears, 
these  internal  and  external  ear-muscles  are  of  great  service 
in  altering  the  shell  of  the  ear,  so  as  to  catch  the  waves  of 
sound  as  much  as  possible.  But  in  the  case  of  man  and 
other  short-eared  mammals  these  muscles  are  useless,  though 
they  are  still  present.  Our  ancestors  having  long  abandoned 
the  use  of  them,  we  cannot  work  them  at  all  to-day.  In  the 
inner  corner  of  the  eye  we  have  a  small  crescent-shaped  fold 
of  skin  ;  this  is  the  last  relic  of  a  third  inner  eye-lid,  called 


THE  MODERN  SCIENCE  OF  EVOLUTION  87 


the  nictitating  (winking)  membrane.  This  membrane  is 
highly  developed  and  o(  great  service  in  some  of  our  distant 
relations,  such  as  fishes  of  the  shark  type  and  several  other 
vertebrates;  in  us  it  is  shrunken  and  useless.  In  the 
intestines  we  have  a  process  that  is  not  only  quite  useless, 
but  may  be  very  harmful — the  vermiform  appendix.  This 
small  intestinal  appendage  is  often  the  cause  of  a  fatal 
illness.  If  a  cherry-stone  or  other  hard  body  is  unfortunately 
squeezed  through  its  narrow  aperture  during  digestion,  a 
violent  inflammation  is  set  up,  and  often  proves  fatal.  This 
appendix  has  no  use  whatever  now  in  our  frame  ;  it  is  a 
dangerous  relic  of  an  organ  that  was  much  larger  and  was  of 
great  service  in  our  vegetarian  ancestors.  It  is  still  large 
and  important  in  many  vegetarian  animals,  such  as  the  apes 
and  the  ungulates. 

There  are  similar  rudimentary  organs  in  all  parts  of  our 
body,  and  in  all  the  higher  animals.  They  are  among  the 
most  interesting  phenomena  to  which  comparative  anatomy 
introduces  us  ;  partly  because  they  furnish  one  of  the  clearest 
proofs  of  evolution,  and  partly  because  they  most  strikingly 
refute  the  teleology  of  certain  philosophers.  The  theory  of 
evolution  enables  us  to  give  a  very  simple  explanation  of 
these  phenomena. 

We  have  to  look  on  them  as  organs  which  have  fallen 
into  disuse  in  the  course  of  many  generations.  With  the 
decrease  in  the  use  of  its  function,  the  organ  itself  shrivels  up 
gradually,  and  finally  disappears.  There  is  no  other  way  of 
explaining  rudimentary  organs.  Hence  they  arc  also  of 
great  interest  in  philosophy;  they  show  clearly  that  the 
monistic  or  mechanical  view  o(  the  organism  is  the  only 
correct  one,  and  that  the  dualistic  or  teleological  conception 
is  wrong.  The  ancient  legend  of  the  direct  creation  of  man 
according  to  a  pre-conceived  plan  and  the  empty  phrases 
about  "design  "  in  the  organism  are  completely  shattered  by 
them.  It  would  lie  difficult  to  conceive  a  more  thorough 
refutation  of  teleology  than  is  furnished  by  the  fact  that  all  the 
higher  animals  have  these  rudimentary  organs. 

Moreover,  in  the  light  of  these  facts  of  dysteleology,  we  see 


THE  MODERN  SCIENCE  OF  EVOLUTION 


the  hollowness  of  the  phrases  about  a  "  moral  government  of 
the  world."  No  one  but  a  learned  idealist  or  a  well-meaning 
optimist  who  shuts  his  eyes  to  facts  can  speak  to-day  of  such 
a  "moral  order."  There  is,  unfortunately,  no  more  trace  of 
it  in  nature  than  in  human  life — no  more  in  natural  history 
than  in  the  history  of  civilisation.  A  grim  and  ceaseless 
struggle  for  life  is  the  real  mainspring  of  the  purposeless 
drama  of  the  world's  history.  We  can  only  see  a  "  moral 
order"  and  "design"  in  it  when  we  ignore  the  triumph  of 
immoral  force  and  the  aimless  features  of  the  organism. 
Might  goes  before  right  as  long  as  organic  life  exists. 

The  theory  of  evolution  finds  its  broadest  inductive 
foundation  in  the  natural  classification  of  living  things,  which 
arranges  all  the  various  forms  in  larger  and  smaller  groups, 
according  to  their  degree  of  affinity.  These  groupings  or 
categories  of  classification — the  varieties,  species,  genera, 
families,  orders,  classes,  etc. — show  such  constant  features  of 
co-ordination  and  subordination  that  we  are  bound  to  look  on 
them  as  genealogical,  and  represent  the  whole  system  in  the 
form  of  a  branching  tree.  This  is  the  genealogical  tree  of 
the  variously  related  groups;  their  likeness  in  form  is  the 
expression  of  a  real  affinity.  As  it  is  impossible  to  explain 
in  any  other  way  the  natural  tree-like  form  of  the  system  of 
organisms,  we  must  regard  it  at  once  as  a  weighty  proof  of 
the  truth  of  evolution.  The  careful  construction  of  these 
genealogical  trees  is,  therefore,  not  an  amusement,  but  the 
chief  task  of  modern  classification. 

Among  the  chief  phenomena  that  bear  witness  to  the 
inductive  law  of  evolution  we  have  the  geographical  distri- 
bution of  the  various  species  of  animals  and  plants  over  the 
surface  of  the  earth,  and  their  topographical  distribution  on 
the  summits  of  mountains  and  in  the  depths  of  the  ocean. 
The  scientific  study  of  these  features — the  "  science  of  distri- 
bution," or  chorology  (chora  =  a  place) — has  been  pursued 
with  lively  interest  since  the  discoveries  made  by  Alexander 
von  Humboldt.  Until  Darwin's  time  the  work  was  confined 
to  the  determination  of  the  facts  of  the  science,  and  chiefly 
aimed  at  settling  the  spheres  of  distribution  of  the  existing 


THE  MODERN  SCIENCE  OF  EVOLUTION 


large  and  small  groups  of  living  things.  It  was  impossible 
at  that  time  to  explain  the  causes  of  this  remarkable  distribu- 
tion, or  the  reasons  why  one  group  is  found  only  in  one 
locality  and  another  in  a  different  place,  and  why  there  is  this 
manifold  distribution  at  all.  Here,  again,  the  theory  of 
evolution  has  given  us  the  solution  of  the  problem.  It 
furnishes  the  only  possible  explanation  when  it  teaches  that 
the  various  species  and  groups  of  species  descend  from 
common  stem-forms,  whose  ever-branching  offspring  have 
gradually  spread  themselves  by  migration  over  the  earth. 
For  each  group  of  species  we  must  admit  a  "  centre  of 
production,"  or  common  home  ;  this  is  the  original  habitat 
in  which  the  ancestral  form  was  developed,  and  from  which 
its  descendants  spread  out  in  every  direction.  Several  of 
these  descendants  became  in  their  turn  the  stem-forms  for 
new  groups  of  species,  and  these  also  scattered  themselves  by 
active  and  passive  migration,  and  so  on.  As  each  migrating 
organism  found  a  different  environment  in  its  new  home,  and 
adapted  itself  to  it,  it  was  modified,  and  gave  rise  to  new 
for  ins. 

This  very  important  branch  of  science  that  deals  with 
active  and  passive  migration  was  founded  by  Darwin,  with 
the  aid  of  the  theory  of  evolution  ;  and  at  the  same  time  he 
advanced  the  true  explanation  of  the  remarkable  chorological 
relation  of  the  living  population  in  any  locality  to  the  fossil 
forms  found  in  it.  Moritz  Wagner  very  ably  developed  his 
idea  under  the  title  of  "the  theory  of  migration."  In  my 
opinion,  this  famous  traveller  has  rather  over-estimated  the 
value  of  his  theory  of  migration  when  he  takes  it  to  be  an 
indispensable  condition  of  the  formation  of  new  species  and 
opposes  the  theory  of  selection.  The  two  theories  are  not 
opposed  in  their  main  features.  Migration  (by  which  the 
stem-form  of  a  new  species  is  isolated)  is  really  only  a  special 
case  of  selection.  The  striking  and  interesting  facts  of 
chorology  can  only  be  explained  by  .the  theory  of  evolution, 
and  therefore  we  must  count  them  among  the  most  important 
of  its  inductive  bases. 

The  same  must  be  said  of  all  the  remarkable  phenomena 


THE  MODERN  SCIENCE  OF  EVOLUTION 


which  we  perceive  in  the  economy  of  the  living  organism. 
The  many  and  various  relations  of  plants  and  animals  to  each 
other  and  to  their  environment,  which  are  treated  in  bionomy 
(the  oecology  or  ethology  of  organisms,  from  nomas,  law  or 
norm,  and  bios,  life),  the  interesting  facts  of  parasitism, 
domesticity,  care  of  the  young,  social  habits,  etc.,  can  only 
be  explained  by  the  action  of  heredity  and  adaptation. 
Formerly  people  saw  only  the  guidance  of  a  beneficent 
Providence  in  these  phenomena  ;  to-day  we  discover  in  them 
admirable  proofs  of  the  theory  of  evolution.  It  is  impossible 
to  understand  them  except  in  the  light  of  this  theory  and  the 
struggle  for  life. 

Finally,  we  must,  in  my  opinion,  count  among  the  chief 
inductive  bases  of  the  theory  of  evolution  the  fcetal  develop- 
ment of  the  individual  organism,  the  whole  science  of 
embryology  or  ontogeny.  But  as  the  later  chapters  will  deal 
with  this  in  detail,  I  need  say  nothing  further  here.  I  shall 
endeavour  in  the  following  pages  to  show,  step  by  step,  how 
the  whole  of  the  embryonic  phenomena  form  a  massive  chain 
of  proof  for  the  theory  of  evolution  ;  for  they  can  be 
explained  in  no  other  way.  In  thus  appealing  to  the  close 
causal  connection  between  ontogenesis  and  phylogenesis, 
and  taking  our  stand  throughout  on  the  biogenetic  law,  we 
shall  be  able  to  prove,  stage  by  stage,  from  the  facts  of 
embryology,  the  evolution  of  man  from  the  lower  animals. 

The  general  adoption  of  the  theory  of  evolution  has 
definitely  closed  the  controversy  as  to  the  nature  or  definition 
of  the  species.  This  question  had  received  a  great  variety  of 
answers  during  the  last  century,  but  no  satisfactory  result 
had  been  reached.  Thousands  of  botanists  and  zoologists 
were  engaged  daily  in  the  classification  and  description  of 
species,  but  they  made  no  progress.  Many  hundreds  of 
thousands  of  animal  and  plant  groups  were  declared  to  be 
"  real  species,"  without  the  authors  being  able  to  give  any 
proof  or  logical  justification  of  their  divisions.  There  were 
endless  controversies  between  the  classifiers  as  to  whether 
the  group  in  question  was  a  true  or  false  species,  a  species 
or  a  variety,  a  sub-species  or  a  race,  though  they  had  never 


THE  MODERN  SCIENCE  OF  EVOLUTION 


asked  themselves  the  real  meaning  of  these  terms.  If  they 
had  striven  to  be  clear  on  this  point,  they  would  have  seen 
long  ago  that  the  words  have  no  absolute  meaning  whatever, 
but  are  only  group-names,  or  categories  of  classification,  with 
a  purely  relative  value. 

In  1857,  it  is  true,  a  famous  and  gifted,  but  inaccurate  and 
dogmatic,  scientist,  Louis  Agassiz,  attempted  to  give  an 
absolute  value  to  these  "categories  of  classification."  He 
did  this  in  his  Essay  on  Classification,  in  which  he  turns 
upside  down  the  phenomena  of  organic  nature,  and,  instead 
of  tracing  them  to  their  natural  causes,  examines  them 
through  a  theological  prism.  The  true  species  (bona  species ) 
was,  he  said,  an  "incarnate  idea  of  the  Creator."  Unfortu- 
nately, this  pretty  phrase  has  no  more  scientific  value  than 
all  the  other  attempts  to  save  the  absolute  or  intrinsic  value 
of  the  species.  I  believe  I  have  shown  this  clearly  enough 
in  the  exhaustive  criticism  of  the  morphological  and  physio- 
logical idea  of  the  species  and  the  categories  of  classification 
which  I  gave  in  my  Generelle  Morphologie  (Band  II.,  SS.  323- 
402).  Agassiz's  "Creator"  is  an  idealised  man,  an  imagi- 
native architect,  who  is  ever  planning  and  producing  new 
species.  (See  also  the  third  chapter  of  the  Natural  History 
of  Creation.) 

The  dogma  of  the  fixitv  and  creation  of  species  lost  its 
last  great  champion  when  Agassiz  died  in  1S73.  The 
opposite  theory,  that  all  the  different  species  descend  from 
common  stem-forms,  encounters  no  serious  difficulty  to-day. 
All  the  endless  research  into  the  nature  of  the  species,  and 
the  possibility  oi  several  species  descending  from  a  common 
ancestor,  has  been  closed  to-day  by  the  removal  of  the  sharp 
limits  that  had  been  set  up  between  species  and  varieties  on 
the  one  hand,  and  species  and  genera  on  the  other.  I  gave 
an  analytic  proof  o(  this  in  my  monograph  on  the  sponges 
(1872),  having  made  a  very  close  study  of  variability  in  this 
small  but  highly  instructive  group, and  shown  the  impossibility 
of  making  any  dogmatic  distinction  of  species.  According 
as  the  classifier  takes  his  ideas  of  genus,  species,  and  variety 
in  a  broader  or  in  a  narrower  sense,  he  will   find   in  the  small 


THE  MODERN  SCIENCE  OF  EVOLUTION 


group  of  the  sponges  either  one  genus  with  three  species,  or» 
three  genera  with  238  species,  or  113  genera  with  59I  species. 
Moreover,  all  these  forms  are  so  connected  by  intermediate 
forms  that  we  can  convincingly  prove  the  descent  of  all  the 
sponges  from  a  common  stem-form,  the  olynthus. 

Here,  I  think,  I   have  given  an  analytic  solution  of  the 
problem  of  the  origin  of  species,  and  so  met  the  demand  of 
certain    opponents   of  evolution    for   an    actual    instance   of 
descent  from  a  stem-form.     Those  who  are  not  satisfied  with 
the  synthetic  proofs  of  the  theory  of   evolution    which    are 
provided  by  comparative  anatomy,  embryology,  paleontology, 
dysteleology,  chorology,  and  classification,  may  try  to  refute 
the  analytic  proof  given  in   my  treatise  on  the  sponge,  the 
outcome  of   five  years  of  assiduous  study.     I   repeat:  It  is 
now  impossible  to  oppose  evolution  on  the  ground  that  we 
have  no  convincing  example  of  the  descent  of  all  the  species 
of  a  group  from  a  common  ancestor.     The   monograph  on 
the  sponges  furnishes  such  a  proof,  and,  in  my  opinion,  an 
indisputable  proof.     Any  man  of  science  who  will  follow  the 
protracted  steps   of  my   inquiry  and   test   my  assertions  will 
find  that  in  the  case  of  the  sponges  we  can  follow  the  actual 
evolution  of  species,  in  static  nascenti.     And  if  this  is  so,  if 
we  can  show  the  origin  of  all   the  species  from  a  common 
form  in  one  single  class,  we  have  the  solution  of  the  problem 
of  man's  origin,  because  we  are  in  a  position  to  prove  clearly 
his  descent  from  the  lower  animals. 

At  the  same  time,  we  can  now  reply  to  the  often-repeated 
assertion,  even  heard  from  scientists  of  our  own  day,  that  the 
descent  of  man  from  the  lower  animals,  and  proximately  from 
the  apes,  still  needs  to  be  "proved  with  certainty."  These 
"  certain  proofs  "  have  been  available  for  a  longtime;  one  has 
only  to  open  one's  eyes  to  see  them.  It  is  a  mistake  to  seek 
them  in  the  discovery  of  intermediate  forms  between  man  and 
the  ape,  or  the  conversion  of  an  ape  into  a  human  being  by 
skilful  education.  The  proofs  lie  in  the  great  mass  of 
empirical  material  we  have  already  collected.  They  are 
furnished  in  the  strongest  form  by  the  data  of  comparative 
anatomy  and  embryology,  completed  by  paleontology.     It  is 


THE  MODERN  SCIENCE  OF  EVOLUTION  93 

not  a  question  now  of  detecting  new  proofs  of  the  evolution 
of  man,  but  of  examining'  and  understanding  the  proofs  we 
already  have. 

It  seems  especially  urgent  to  refer  to-day  to  these  various 
sources  of  phylogeny,  and  point  out  how  they  confirm  each 
other,  because  the  growth  of  specialism  in  every  branch  of 
biology  and  the  enormous  accumulation  of  fresh  observations 
in  detail  have  led  to  a  certain  amount  of  narrowness  in  appre- 
ciating them.  Many  modern  embryologists  occupy  themselves 
with  the  application  of  their  improved  methods  to  the  detailed 
study  of  minute  sections  of  the  embryo  and  the  mechanical 
analysis  of  them,  and  fail  to  keep  in  view  the  entire  organism 
and  its  important  relations  to  others  of  the  same  stem,  as 
shown  in  comparative  anatomy  and  classification.  Many  of 
the  misleading  theories  of  this  modern  mechanical  embryo- 
logy would  never  have  been  formulated  if  their  authors  had 
been  acquainted  with  the  relevant  facts  of  paleontology.  On 
the  other  hand,  however,  most  of  the  paleontologists  are 
ignorant  of  the  most  important  results  of  comparative 
embryology,  and  so  fail  to  appreciate  the  value  of  the  bio- 
genetic law.  However  important  it  is  to  determine  the  facts 
of  paleontology  accurately,  their  evolutionary  significance 
cannot  be  properly  appraised  without  the  aid  of  comparative 
anatomy  and  ontogeny.  At  the  same  time,  workers  in  these 
latter  sciences  must  never  lose  touch  with  the  results  of 
paleontology.  Comparative  anatomists  will  reach  no  satis- 
factory result  if  they  seek  to  determine  the  homologies  and 
affinities  of  animal  forms  merely  by  a  comparison  of  living 
species,  without  any  regard  to  their  extinct  ancestors.  The 
distinguished  New  York  paleontologist,  Henry  Osborn,  has 
recently  laid  stress  on  the  wisdom  of  basing  the  science  of 
evolution  on  a  comprehensive  use  of  all  the  three  sources  of 
evidence.  Our  science  requires  these  three  supports  as  much 
as  the  stool  needs  its  three  legs. 

I  was  almost  alone  thirty-six  years  ago  when  I  made  the 
first  attempt,  in  my  Generellc  Morpliologie,  to  put  organic 
morphology  on  a  mechanical  foundation  through  Darwin's 
theory    of     descent.       The    association    of    ontogeny    and 


94  THE  MODERN  SCIENCE  OF  EVOLUTION 

phylogeny  and  the  proof  of  the  intimate  causal  connection 
between  these  two  sections  of  the  science  of  evolution,  which 
I  expounded  in  my  work,  met  with  the  most  spirited  opposi- 
tion on  nearly  all  sides.  The  next  ten  years  were  a  terrible 
"struggle  for  life"  for  the  new  theory.  But  for  the  last 
twenty-five  years  the  tables  have  been  turned.  The  phyloge- 
netic  method  has  met  with  so  general  a  reception,  and  found 
so  prolific  a  use  in  every  branch  of  biology,  that  it  seems 
superfluous  to  treat  any  further  here  of  its  validity  and  results. 
The  proof  of  it  lies  in  the  whole  morphological  literature  of 
the  last  three  decades.  But  no  other  science  has  been  so 
profoundly  modified  in  its  leading  thoughts  by  this  adoption, 
and  been  forced  to  yield  such  far-reaching  consequences,  as 
that  science  which  I  am  now  seeking  to  establish — monistic 
anthropogeny. 

This  statement  may  seem  to  be  rather  audacious,  since  the 
very  next  branch  of  biology,  anthropology  in  the  stricter 
sense,  makes  very  little  use  of  these  results  of  anthro- 
pogeny, and  sometimes  expressly  opposes  them.  This 
applies  especially  to  the  attitude  which  has  characterised 
the  German  Anthropological  Society  (the  Deutsche  Gesell- 
schaftfiir  Autliropologie)  for  some  thirty  years.  Its  powerful 
president,  the  famous  pathologist,  Rudolph  Virchow,  is 
chiefly  responsible  for  this.  Until  his  death  (September  5th, 
1902)  he  never  ceased  to  reject  the  theory  of  descent  as 
unproven,  and  to  ridicule  its  chief  consequence — the  descent 
of  man  from  a  series  of  mammal  ancestors — as  a  fantastic 
dream.  I  need  only  recall  his  well-known  expression  at  the 
Anthropological  Congress  at  Vienna  in  1894,  that  "  it  would 
be  just  as  well  to  say  man  came  from  the  sheep  or  the 
elephant  as  from  the  ape." 

Virchow's  assistant,  the  secretary  of  the  German  Anthro- 
pological Society,  Professor  Johannes  Ranke  of  Munich,  has 
also  indefatigably  opposed  transformism  :  he  has  succeeded 
in  writing  a  work  in  two  volumes  ( Der  Mensch),  in  which  all 
the  facts  relating  to  his  organisation  are  explained  in  a  sense 
hostile  to  evolution.  This  work  has  had  a  wide  circulation, 
owing-  to  its  admirable  illustrations  and  its  able  treatment  of 


Till-:  MODERN  SCIENCE  OF  EVOLUTION  95 

the  most  interesting  facts  of  anatomy  and  physiology — 
exclusive  o(  the  sexual  organs!  But,  as  it  has  done  a  great 
deal  to  spread  erroneous  views  among  the  general  public,  I 
have  included  a  criticism  o(  it  in  mv  Natural  History  of 
Creation,  as  well  as  met  Virchow's  attacks  on  anthropogeny. 

Neither  Virchow,  nor  Ranke,  nor  any  other  "exact" 
anthropologist,  has  attempted  to  give  any  other  natural 
explanation  o(  the  origin  of  man.  Thev  have  either  set 
completely  aside  this  "question  of  questions"  as  a  tran- 
scendental problem,  or  they  have  appealed  to  religion  for  its 
solution.  We  have  to  show  that  this  rejection  of  the  rational 
explanation  is  totally  without  justification.  The  fund  of 
knowledge  which  has  accumulated  in  the  progress  of  biology 
in  the  nineteenth  century  is  quite  adequate  to  furnish  a 
rational  explanation,  and  to  establish  the  theory  of  the 
evolution  ol    man  on  the  solid  facts  of  his  embryology. 


CHAPTER   VI. 

THE   OVUM    AND   THE   AMCEBA1 

The  ovum  of  man  and  other  animals  is  a  simple  cell.  The  fully-developed 
man  is  an  organised  community  of  cells.  Independent  cells  and  tissue- 
cells.  Importance  and  chief  features  of  the  cell  theory.  Definition,  form, 
and  size  of  the  cell.  Consists  of  two  parts  :  Nucleus  (caryoplasm)  and 
cell-body  (cytosoma — cytoplasm).  Active  protoplasm  and  passive  products 
of  protoplasm.  The  cell  as  the  elementary  organism,  or  the  unit-individual. 
Plastids,  or  constructive  cells.  Their  vital  phenomena.  Vegetal  functions 
(nutrition,  reproduction).  Animal  functions  (movement,  sensation).  The 
special  features  of  the  ovum.  Yelk.  Germinal  vesicles.  Germinal  disc. 
Coverings  of  the  ovum,  ovolemma  or  chorion.  Application  of  the  biogenetic 
law  to  the  ovum.  Unicellular  organisms.  The  amoeba.  Structure  and 
functions  of  the  amoeba.  Amoeboid  movements.  Amoeboid  cells  in  the 
multicellular  organism.  Their  movements  and  intussusception  of  ^olid 
matter.  Blood-cells  that  eat.  Comparison  of  the  amoeba  with  the  ovum. 
Amoeboid  ova  of  the  sponges  and  their  movements.  Evolutionary  con- 
clusion from  the  unicellular  ovum  to  the  unicellular  ancestor. 

In  order  to  understand  clearly  the  course  of  human  embryo- 
logy, we  must  select  the  more  important  of  its  wonderful  and 
manifold  processes  for  fuller  explanation,  and  then  proceed 
from  these  to  the  innumerable  features  of  less  importance. 
The  most  important  feature  in  this  sense,  and  the  best 
starting-point  for  ontogenetic  study,  is  the  fact  that  man  is 
developed  from  an  ovum,  and  that  this  ovum  is  a  simple 
cell.  The  human  ovum  does  not  materially  differ  in  form 
and  composition  from  that  of  the  other  mammals,  whereas 
there  is  a  distinct  difference  between  the  fertilised  ovum  of  the 
mammal  and  that  of  any  other  animal. 

This  fact  is  so  important  that  few  should  be  unaware  of  its 
extreme  significance  ;  yet  it  was  quite  unknown  in  the  first 
quarter  of  the  nineteenth  century.  As  we  have  seen,  the 
human  and  mammal  ovum  was  not  discovered  until  1827,  when 
Carl  Ernst  von  Baer  detected  it.  Up  to  that  time  the  larger 
vesicles,    in    which    the    real   and    much    smaller    ovum    is 

1  Cf.  Edmund  Wilson,  The  Cell  in  Development  and  Inheritance. 
96 


the  ovr.M  A.xn  the  a M(E ha 


contained,  had  been  wrongly  regarded  as  ova.  The 
important  circumstance  that  this  mammal  ovum  is  a  simple 
cell,  like  the  ovum  of  other  animals,  could  not,  of  course,  be 
recognised  until  the  cell  theory  was  established.  This  was 
not  done,  by  Schleiden  for  the  plant  and  Schwann  for  the 
animal,  until  1838.  As  we  have  seen,  this  cell  theory  is  of 
the  greatest  service  in  explaining  the  human  frame  and  its 
embryonic  development.  Hence  we  must  say  a  few  words 
about  the  actual  condition  of  the  theory  and  the  significance 
of  the  \iews  it  has  suggested. 

In  order  properly  to  appreciate  the 
cellular  theory,  the  most  important  ele- 
ment in  our  morphological  and  physio- 
logical science,  it  is  necessary  to  under- 
stand in  the  first  place  that  the  cell  is  a 
unified  organism,  a   self-contained    living 

beinsj.       When    we    anatomically   dissect     ,    „FlG-      '"—  Tne 
&  -  human  ovum,  mag- 

the  fullv-formed  animal  or   plant  into  its     nified  100  times.    The 

.  globular  mass  of  yelk 

various   organs,    and    then    examine   the     \/,)  is  enclosed  by  a 

finer  structure  of  these  organs  with  the  ^am^o^lem™ 
microscope,  we  are  surprised  to  find  that     or  zona  pellucida  [a]), 

and   contains   a    non- 
all    these    different    parts    are    ultimately     central  nucleus   (the 
,  r     ,  .      ,  gferminal    vesicle,    c\. 

made   up  of  the  same  structural  element     Cim  plg_  t/. 

or  unit.  This  common  unit  of  structure 
is  the  cell.  It  does  not  matter  whether  we  thus  dissect  a 
leaf,  flower,  or  fruit,  or  a  bone,  muscle,  gland,  or  bit 
of  skin,  etc.;  we  find  in  every  case  the  same  ultimate 
constituent,  which  has  been  called  the  cell  since  Schleiden's 
discovery.  There  are  many  opinions  as  to  its  real  nature, 
but  the  essential  point  in  our  view  of  the  cell  is  to  look 
upon  it  as  a  self-contained  or  independent  living  unit. 
It  is,  in  the  words  of  Briicke,  "an  elementary  organism," 
or,  as  Virchow  puts  it,  "a  vital  focus,"  a  "biomeron." 
We  may  define  it  most  precisely  as  the  ultimate 
organic  unit,  or  "an  individual  of  the  first  class";  and 
as  the  cells  are  the  sole  active  principles  in  every  vital 
function,  we  may  call  them  the  "  plastids,"  or  "forma- 
tive   elements"    (cf.     the    Gen.    Morph.,    Band    I.,  S   269). 


gS  THE  OVUM  AXD  THE  AMCEBA 

This  unity  is  found  in  both  the  anatomic  structure  and  the 
physiological  function.  In  the  case  of  the  protists,  the 
entire  organism  usually  consists  of  a  single  autonomous  cell 
throughout  life.  But  in  the  histonal  (tissue-forming) 
animals  and  plants,  which  are  the  great  majority,  the 
organism  begins  its  career  as  a  simple  cell,  and  then  grows 
into  a  cell-community,  or,  more  correctly,  an  organised  cell- 
state.  Our  own  body  is  not  really  the  simple  unity  that  it  is 
generally  supposed  to  be.  On  the  contrary,  it  is  a  very 
elaborate  social  system  of  countless  microscopic  organisms,  a 
colony  or  commonwealth,  made  up  of  innumerable  independent 
units,  or  very  different  tissue-cells. 

In  reality,  the  term  "cell,"  which  existed  long  before  the 
cell  theory  was  formulated,  is  not  happily  chosen.  Schleiden, 
who  first  brought  it  into  scientific  use  in  the  sense  of  the  cell 
theory,  gave  this  name  to  the  elementary  organisms  because, 
when  you  find  them  in  the  dissected  plant,  they  generally 
have  the  appearance  of  chambers,  like  the  cells  in  a  bee-hive, 
with  firm  walls  and  a  fluid  or  pulpy  content.  This  idea  of  a 
cell  as  a  closed  vesicle  or  little  sac,  with  a  fluid  content  and 
firm  envelope  or  wall,  was  adopted,  and  came  into  general 
use  ;  but  it  is  totally  inapplicable  to  most  of  the  cells  in  the 
body.  The  more  we  learned  about  the  cells  of  the  animal 
body,  the  more  it  became  necessary  to  modify  our  concep- 
tion of  the  cell  ;  for  some  cells,  especially  young  ones,  are 
entirely  without  the  enveloping  membrane,  or  stiff  wall. 
Hence  we  now  generally  describe  the  cell  as  a  living,  viscous 
particle  of  protoplasm,  enclosing  a  firmer  nucleus  in  its 
albuminoid  body.  There  maybe  an  enclosing  membrane,  as 
there  actually  is  in  the  case  of  most  of  the  plants  ;  but  it  may 
be  wholly  lacking,  as  is  the  case  with  most  of  the  animals. 
There  is  no  membrane  at  all  in  the  first  stage.  The  young 
cells  are  usually  round,  but  they  vary  much  in  shape  later  on. 
Illustrations  of  this  will  be  found  in  the  cells  of  various  parts 
of  the  body  shown  in  Figs.  3-7. 

Hence  the  essential  point  in  the  modern  idea  of  the  cell  is 
that  it  is  made  up  of  two  different  active  constituents — an 
inner  and  an  outer  part.     The  smaller  and  inner  part  is  the 


THE  OVUM  AND   THE  AM  (EISA 


nucleus  (or  catyon,  or  cylob/astus,  Fig.  ic  and  Fig.  2k).  The 
outer  and  larger  part,  which  encloses  the  other,  is  the  body  of 
the  cell  (celleus,  cytos,  or  cytosoma).  The  soft  living  sub- 
stance of  which  the  two  are  composed  has  a  peculiar  chemical 
composition,  and  belongs  to  the  group  of  the  albuminoid 
plasma-substances  ("formative  matter"),  or  protoplasm. 
The  essential  and  indispensable  element  of  the  nucleus  is  the 
nuclei n  (or  caryoplasm)  ;  that  of  the  cell  body  is  called  the 
plastin  (or  cytoplasm).  In  the  most  rudimentary  cases  both 
substances  seem  to  be  quite 
simple  and  homogeneous,  without 
anv  visible  structure.  But,  as 
a  rule,  when  we  examine  them 
under  a  high  power  of  the  micro- 
scope, we  find  a  certain  structure 
in  the  protoplasm.  The  chief 
and  most  common  form  of  this 
is  the  fibrous  or  net-like  "thread- 
structure  "  (Frommann)  and  the 
frothy  "honeycomb  structure" 
(Butschli). 

The    shape   or    outer    form    of 
the    cell    is    infinitely   varied,    in 

accordance  with  its  endless  power  of  adapting  itself  to  the 
most  diverse  activities  or  environments.  In  its  simplest  form 
the  cell  is  globular  (Fig.  2).  This  normal  globular  form  is 
especially  found  in  cells  of  the  simplest  construction,  and 
those  that  are  developed  in  a  free  fluid  without  any  external 
pressure.  In  such  cases  the  nucleus  also  is  not  infrequently 
round,  and  located  in  the  centre  of  the  cell-body  (Fig.  2k). 
In  other  cases,  the  cells  have  no  definite  shape  ;  they  are 
constantly  changing  their  form  owing  to  their  automatic 
movements.  This  is  the  case  with  the  amcebce  (Figs.  15 
and  16)  and  the  amoeboid  travelling  cells  (Fig.  n),  and  also 
with  very  young  ova  (Fig.  12).  However,  as  a  rule,  the  cell 
assumes  a  definite  form  in  the  course  of  its  career.  In  the 
tissues  of  the  multicellular  organism,  in  which  a  number  of 
similar  cells  are  bound  together  in  virtue  of  certain   laws  of 


Fig.  2.  -Stem-cell  of  one 
of  the  eehinoderms  (cytula,  or 
■first  segmentation-cell"  =ferti- 
lised  ovum  I,  alter  Herhoig.  K 
is  tlii*  nucleus  or  carvon. 


THE  OVUM  AND  THE  AMCEBA 


heredity,  the  shape  is  determined  partly  by  the  form  of  their 
connection  and  partly  by  their  special  functions.  Thus,  for 
instance,  we  find  in  the  mucous  lining  of  our  tongue  very 
thin  and  delicate  flat  cells,  or  epithelial  cells,  of  roundish 
shape  (Fig.  3).  In  the  outer  skin  we  find  similar,  but  harder, 
covering  cells,  joined  together  by  saw-like  edges  (Fig.  4). 
In  the  liver  and  other  glands  there  are  thicker  and  softer  cells, 
linked  together  in  rows  (Fig.  5). 

The  last-named  tissues  (Figs.  3-5)  belong  to  the  simplest 
and  most  primitive  type,  the  group  of  the  "  covering-tissues," 
or  epithelia.  In  these  "primary  tissues"  (to  which  the 
germinal  layers  belong)  simple  cells  of   the  same  kind  are 


Fig.  3. 


Fig.  4. 


Fig.  5. 


Fig.  3. — Three  epithelial  cells  from  the  mucous  lining  of  the  tongue. 
Fig.  4.— Five    spiny  OF  grooved    cells,    with    edges  joined,    from   the 
outer  skin  (epidermis) :  one  of  them  (b)  is  isolated. 

Fig.  5. — Ten  liver-cells  :  one  of  them  (b)  has  two  nuclei. 

arranged  in  layers.  The  arrangement  and  shape  are  more 
complicated  in  the  "  secondary  tissues,"  which  are  gradually 
developed  out  of  the  primary,  as  in  the  tissues  of  the  muscles," 
nerves,  bones,  etc.  In  the  bones,  for  instance,  which  belong 
to  the  group  of  supporting  or  connecting  organs,  the  cells 
(Fig.  6)  are  star-shaped,  and  are  joined  together  by  numbers 
of  net-like  interlacing  processes  ;  so,  also,  in  the  tissues  of 
the  teeth  (Fig.  7),  and  in  other  forms  of  supporting-tissue,  in 
which  a  soft  or  hard  substance  (intercellular  matter,  or  base) 
is  inserted  between  the  cells. 

The  cells  also  differ  very  much  in  size.  The  great 
majority  of  them  are  invisible  to  the  naked  eye,  and  can  be  seen 
only  through  the  microscope  (being  on  an  average  between 


THE  Orr.U  AND  THE  AMCEBA 


o.oi  and  o.i  millimetres  in  diameter).  There  are,  however, 
many  o(  the  smaller  plastids — such  as  the  famous  bacteria — 
which  only  come  into  view  with  a  very  high  magnifying 
power.  On  the  other  hand,  many  cells  attain  a  considerable 
size, and  run  to  several  millimetres  or  centimetres  in  diameter, 
as  do  several  kinds  of  rhizopods  among  the  unicellular  protists 
(such  as  the  radiolaria  and  thalamophora).  Among  the 
tissue-cells  of  the  animal  body  many  of  the  muscular  fibres  and 
nerve  fibres  are  more  than  a  decimetre  (4  inches),  and  some- 
times more  than  a  metre  (40  inches)  in  length.     Among  the 


Fig.  6.— Nine  Star-Shaped  bone-Cells,  with  interlaced  branches. 

largest  cells  are  the  yelk-filled  ova;  as,  for  instance,  the 
yellow  "yelk-nucleus"  in  the  hen's  egg,  which  we  shall 
describe  later  (Fig.  15). 

Cells  also  vary  considerably  in  structure.  In  this  con- 
nection we  must  first  distinguish  between  the  active  and 
passive  components  of  the  cell.  It  is  only  the  former,  or 
active  parts  of  the  cell,  that  really  live,  and  effect  that  marvel- 
lous world  of  phenomena  to  which  we  give  the  name  of 
"organic  life."  The  first  of  these  is  the  inner  nucleus 
(caryoplasmaji  and  the  second  the  body  of  the  cell 
(cytoplasma).     The  passive   portions  come  third  ;   these  are 


THE  OVUM  AND  THE  AMCEBA 


subsequently  formed  from  the  others,  and  I  have  in  my 
Generelle  Morphologie  (chap,  ix.)  given  them  the  name  of 
"  plasma-products."  They  are  partly  external  (cell-membranes 
and  intercellular  matter)  and  partly  internal  (cell-sap  and  cell- 
contents).     (See  the  table  at  the  end  of  the  next  Chapter.) 

The  nucleus  (or  car- 
yon),  which  is  usually 
of  a  simple  roundish 
form,  is  quite  structure- 
less at  first  (especially 
in  very  young  cells),  and 
composed  of  homogene- 
ous nuclear  matter  or 
caryoplasm     (Fig.    zk). 

But,  as  a  rule,  it  forms  a 
Fig.  7.— Eleven     star-shaped    cells  .        ... 

from  the  enamel  of  a  tooth,  joined  together  SOrt  Or  vesicle  later  On,  in 
by  their  branchlets.  i   •    i_  j-    <-•     „    -„i, 

which  wecandistinguisn 
a  more  solid  nuclear  base  (caryobasis)  and  a  softer  or  fluid 
nuclear  sap  (  caryolynnph  ).  The  nuclear  base  forms  the  enve- 
loping membrane  of  globular  nuclein  and,  as  a  rule,  a  skeleton 
or  network  of  branching  threads,  which  go  out  from  the 
membrane,  and  pass  through  the  cavity  of  the  vesicle  and  its 
liquid  contents.  This  nuclear  skeleton  (caryomitoma J  con- 
sists of  two  different  substances,  one  of  which  (the  chromatin ) 
is  strongly  tinged  with  carmine  and  other  colouring  matter, 
and  the  other  / 'achromia  or  lininj  is  not.  In  a  mesh  of  the 
nuclear  network  (or  it  may  be  on  the  inner  side  of  the  nuclear 
envelope)  there  is,  as  a  rule,  a  dark,  very  opaque,  solid  body, 
called  the  nucleolus.  Many  of  the  nuclei  contain  several  of 
these  nucleoli  (as,  for  instance,  the  germinal  vesicle  of  the 
ova  of  fishes  and  amphibia). 

Recently  a  very  small,  but  particularly  important,  part  of 
the  nucleus  has  been  distinguished  as  the  central  body 
(centrosoma) — a  tiny  particle  that  is  originally  found  in  the 
nucleus  itself  (as  in  the  case  of  many  spermacytes,  carcinom- 
cells,  etc.),  but  is  usually  outside  it,  in  the  cytoplasm  ;  as  a 
rule,  fine  threads  stream  out  from  it  in  the  cytoplasm.  From 
the  position    of    the   centrosoma  with    regard    to   the   other 


THE  OVru  AND  THE  AMCEBA 


parts    it   seems  probable   that    it   has   a    high    physiological 

importance  as   a   centre  of    movement  ;    but   it   is  lacking    in 
many  cells. 

The  cell-body  (celleus  or  cytosoma)  also  consists  origi- 
nally, and  in  its  simplest  form,  o(  a  homogeneous  viscid 
plasmic  matter  (cytoplasm ).  But,  as  a  rule,  only  the  smaller 
part  of  it  is  formed  of  the  living  active  cell-substance  (proto- 
plasm) ;  the  greater  part  consists  of  dead,  passive  plasma- 
products  (metaplasma).  It  is  useful  to  distinguish  between 
the  inner  and  outer  of  these.  External  plasma-products 
(which  are  thrust  out  from  the  pro- 
toplasm as  solid  "  structural  matter  ") 
are  the  cell-membranes  and  the  in- 
tercellular matter.  The  internal 
plasma-products  are  either  the  fluid 
cell-sap  ( cy/o/ymph )  or  hard  struc- 
tures (paraplasma).  As  a  rule,  in 
mature  and  differentiated  cells  these 
various  parts  are  so  arranged  that 
the  protoplasm  (like  the  caryoplasm  fig.  8. -Unfertilised  ovum 
in  the  vesicular  nucleus)  forms  a  sort    °f    an    eehinoderm    (from 

HertwigJ.       I  he    vesicular 

of    skeleton   or  frame-work  (cytonii-    nucleus  (or  "germinal  vesicle") 

,-,  ...       is    globular,   halt'  the   size   of 

toma,  hlar  matter  or  spongioplasm).     the  round  ovum,  and  encloses 

The  spaces  of  this  network  are  filled    a  "uc,1e"  f^i^i  '"  ?f 

t  central  knot  ot  which  there  is  a 

partlv  with   the   fluid   cell-sap  ( cv/a-    dark  nucleolus  (the  "germinal 
'  r  l    '  spot  "). 

lymph)  and  partly  by  hard  struc- 
tural products  (paraplasma,  or  interfilar  matter)  ;  among 
these  there  are  small  plasma-granules  (granula  or  micro- 
somata),  or  fat-grains  (liposomata),  of  great  importance. 
Besides  these,  we  can  distinguish  many  other  products  in  the 
cytoplasm,  such  as  concrementa,  crystals,  gland-granules,  etc. 
The  simple  globular  ovum,  which  we  take  as  the  starting- 
point  of  our  study  (Figs,  i  and  2),  has  in  many  cases  the 
vague,  indifferent  features  of  the  typical  primitive  cell.  As 
a  contrast  to  it,  and  as  an  instance  of  a  very  highly  differen- 
tiated plastid,  we  may  consider  for  a  moment  a  large  nerve- 
cell,  or  ganglionic  cell,  from  the  brain.  The  ovum  stands 
potentially  for  the  entire  organism — in  other  words,  it  has  the 


THE  O  VUM  AND  THE  AMCEBA 


faculty  of  building  up  out  of  itself  the  whole  multicellular 
body.  It  is  the  common  parent  of  all  the  countless  generations 
of  cells  which  form  the  different  tissues  of  the  body ;  it 
unites  all  their  powers  in  itself,  though  only  potentially  or  in 
germ.  In  complete  contrast  to  this,  the  neural  cell  in  the 
brain  (Fig.  9)  developes  along  one  rigid  line.  It  cannot,  like 
the  ovum,  beget  endless  generations  of  cells,  of  which  some 
will  become  skin-cells,  others  muscle-cells,  and  others  again 
bone-cells.  But,  on  the  other  hand,  the  nerve-cell  has 
become  fitted  to  discharge  the  highest  functions  of  life  ;  it 
has  the  powers  of  sensation,  will,  and  thought.  It  is  a  real 
soul-cell,  or  an  elementary  organ  of  the  psychic  activity. 
It  has,  therefore,  a  most  elaborate  and  delicate  structure. 
Numbers  of  extremely  fine  threads,  like  the  electric  wires 
at  a  large  telegraphic  centre,  cross  and  recross  in  the  delicate 
protoplasm  of  the  nerve-cell,  and  pass  out  in  the  branching 
processes  which  proceed  from  it  and  put  it  in  communication 
with  other  nerve-cells  or  nerve-fibres  (a,  b).  We  can  only 
partly  follow  their  intricate  paths  in  the  fine  nucleolar  matter 
of  the  cytoplasmic  body.       \y 

Here  we  have  a  most  elaborate  apparatus,  the  delicate 
structure  of  which  we  are  just  beginning  to  appreciate  through 
our  most  powerful  microscopes,  but  whose  significance  is 
rather  a  matter  of  conjecture  than  knowledge.  Its  intricate 
structure  corresponds  to  the  very  complicated  functions  of  the 
mind.  Nevertheless,  this  elementary  organ  of  psychic 
activity — of  which  there  are  thousands  in  our  brain — is 
nothing  but  a  single  cell.  Our  whole  mental  life  is  only  the 
joint  result  of  the  combined  activity  of  all  these  nerve-cells,  or 
soul-cells.  In  the  centre  of  each  cell  there  is  a  large  transparent 
nucleus,  containing  a  small  and  dark  nuclear  body.  Here,  as 
elsewhere,  it  is  the  nucleus  that  determines  the  individuality 
of  the  cell  ;  it  proves  that  the  whole  structure,  in  spite  of  its 
intricate  composition,  amounts  to  only  a  single  cell. 

In  contrast  with  this  very  elaborate  and  very  strictly 
differentiated  psychic  cell  (Fig.  9),  we  have  our  ovum 
(Figs.  1  and  2),  which  has  hardly  any  structure  at  all. 
But  even  in   the  case   of    the   ovum  we  must  infer  from    its 


THE  OVUM  AND  THE  AMCEBA 


Fig.  9.—  A  large  branching  nerve-cell,  or  "soul-cell,"    from  the 

brain  of  an  electric  fish  (torpedo),  magnified  600  times.  In  the  middle  of  the 
cell  is  the  large  transparent  round  nucleus,  one  nucleolus,  and,  within  the  latter 
again,  a  nucleolinus.  The  protoplasm  ol  the  cell  is  split  into  innumerable  fine 
threads  (or  fibrils),  which  are  embedded  in  nucleolar  intercellular  matter,  and 
are  prolonged  into  the  branching  processes  of  the  cell  (b).  One  branch  (a) 
pas^e-  into  a  nerve-fibre.     (Krom  Max  Schultee.) 


THE  OVUM  AND  THE  AMCEBA 


properties  that  its  protoplasmic  body  has  a  very  complicated 
chemical  composition  and  a  fine  molecular  structure  which 
escapes  our  observation.  This  hypothetical  molecular  struc- 
ture of  the  plasm  is  now  generally  admitted  ;  but  it  has  never 
been  seen,  and,  indeed,  lies  far  beyond  the  range  of  micro- 
scopic vision.  It  must  not  be  confused — as  is  often  done — 
with  the  structure  of  the  plasma  (the  fibrous  net-work,  groups 
of  granules,  honey-comb,  etc.)  which  does  come  within  the 
range  of  the  microscope. 

But  when  we  speak  of  the  cells  as  the  elementary 
organisms,  or  structural  units,  or  "  ultimate  individualities," 
we  must  bear  in  mind  a  certain  restriction  of  the  phrases.  I 
mean,  that  the  cells  are  not,  as  is  often  supposed,  the  very 
lowest  stage  of  organic  individuality.  There  are  yet  more 
elementary  organisms  to  which  I  must  refer  occasionally,  and 
will  return  later  on.  These  are  what  we  call  the  "  cytodes  " 
(cytos  =  cell),  certain  living,  independent  beings,  consisting 
only  of  a  particle  of  plasson — an  albuminoid  substance,  which 
is  not  yet  differentiated  into  caryoplasm  and  cytoplasm,  but 
combines  the  properties  of  both.  Those  remarkable  beings 
called  the  monera — especially  the  chromacea  and  bacteria — 
are  specimens  of  these  simple  cytodes.  (Compare  the 
nineteenth  Chapter.)  To  be  quite  accurate,  then,  we  must 
say  :  the  elementary  organism,  or  the  ultimate  individual,  is 
found  in  two  different  stages.  The  first  and  lower  stage  is 
the  cytode,  which  consists  merely  of  a  particle  of  plasson,  or 
quite  simple  plasm.  The  second  and  higher  stage  is  the 
cell,  which  is  already  divided  or  differentiated  into  nuclear 
matter  and  cellular  matter.  We  comprise  both  kinds — the 
cytodes  and  the  cells — under  the  name  of  plastids  ("  formative 
particles "),  because  they  are  the  real  builders  of  the 
organism.  However,  these  cytodes  are  not  found,  as  a  rule, 
in  the  higher  animals  and  plants;  here  we  have  only  real 
cells  with  a  nucleus.  Hence,  in  these  tissue-forming 
organisms  (both  plants  and  animal)  the  organic  unit  always 
consists  of  two  chemically  and  anatomically  different  parts — the 
outer  cell-body  ( cytosoma)  and  the  inner  nucleus  (  caryon ). 

In  order  to  convince  oneself  that   this  cell    is  really  an 


THE  OVUM  AXD  THE  AMCEBA 


independent  organism,  we  have  only  to  observe  the  develop- 
ment and  vital  phenomena  of  one  of  them.  You  see  then 
that  it  performs  all  the  essential  functions  of  life — both 
vegetal  and  animal  —which  we  find  in  the  entire  organism. 
Each  of  these  tiny  beings  grows  and  nourishes  itself  indepen- 
dently. It  takes  its  food  from  the  surrounding  fluid  ; 
sometimes,  even,  the  naked  cells  take  in  solid  particles  at 
certain  points  of  their  surface — in  other  words,  "  eat"  them — 
without  needing  any  special  mouth  and  stomach  for  the 
purpose  (cf.  Fig.  19). 

Further,  each  cell  is  able  to  repro- 
duce itself.  This  multiplication,  in  most 
cases,  takes  the  form  of  a  simple  cleavage, 
sometimes  direct,  sometimes  indirect  ; 
the  simple  direct  (or  "  amitotic  ")  division 
is  less  common,  and  is  found,  for  in- 
stance, in  the  blood  cells  (Fig.  10).  In 
these  the  nucleus  first  divides  into  two 
equal  parts  by  constriction.  The  indirect 
(or  "  mitotic ")  cleavage  is  much  more 
frequent ;  in  this  the  caryoplasm  of  the 
nucleus  and  the  cytoplasm  of  the  cell- 
body  act  upon  each  other  in  a  peculiar 
way,  with  a  partial  dissolution  1  caryo- 
lysisj,  the  formation  of  knots  and  loops 
f  mitosis),  and  a  movement  of  the  halved 
plasma-particles  towards  two  mutually 
repulsive  poles  of  attraction  (caryokine- 
sis,  Fig.  1 1). 

The  intricate  physiological  processes 
which  accompany  this  "mitosis"  have 
been  very  closely  studied  of  late  years.  The  inquiry 
has  led  to  the  detection  of  certain  laws  of  evolution 
which  are  of  extreme  importance  in  connection  with 
heredity.  As  a  rule,  two  very  different  parts  of  the 
nucleus  play  an  important  part  in  these  changes.  They  are  : 
the  chromatin,  or  coloured  nuclear  substance,  which  has  a 
peculiar    property    of     tinging    itself    deeply    with    certain 


Fig.  10.  —  Blood- 
cells,  multiplying  by 
direct  division,  from 

the  blood  of  the  embryo 
of  a  goat.  Originally, 
each  blood-cell  has  a 
nucleus  and  is  globular 
(a).  When  it  is  going 
to  multiply,  the  nucleus 
divides  into  two  (A,  r,  d). 
Then  the  protoplasmic 
body  is  constricted  be- 
tween the  two  nuclei, 
and  these  move  away 
from  each  other  I  el. 
Finally,  the  constriction 
is  complete,  and  the  cell 
splits  into  two  daughter- 
cells  (/).     ( From  Frey.  \ 


THE  OVUM  AND  THE  AMCEBA 


colouring  matters  (carmine,  haematoxylin,  etc.),  and  the 
achromin  (or  linin,  or  achromatin),  a  colourless  nuclear 
substance  that  lacks  this  property.  The  latter  generally 
forms  in  the  dividing  cell  a  sort  of  spindle,  at  the  poles  of 
which  there  is  a  very  small  particle,  also  colourless,  called 
the  "central  body  "  (centrosoma).     This  acts  as  the  centre  or 

_,    Nuclear  threads  (chromo- 

somata) 
i  (coloured  nuclear  matter, 

\  chromatin) 

I  , 

A — •  Nuclear  membrane 

/ ■  Nuclear  sap 


A.  Mother-cell       cyto--  „ 

soma    (,t 
(Knot,  spirema) 

Protoplasm  of 
the  cell-body 


B.  Mother-star, 

the  loops  beginning'  to  split 
lengthways     (nuclear    mem- 
brane gone). 


C.  The  two  daughter- 
stars, 

produced  by  the  breaking  of 

the  loops  of  the  mother-star 

(moving  away). 


D.  The  two  daughter- 
cells, 

produced    by    the    complete 
division    of  the  two   nuclear 
halves  (cytosomata  still  con- 
nected at  the  equator) 
(Double-knot,  Dispirema) 


—  Star-like  appearance  in  cytoplasm 
r —  Centrosoma(sphere  of  attraction) 
A—  Nuclear    spindle    (achromin, 
Hj         colourless  matter) 


'/      Nuclear     loops     (chromatin, 
coloured  matter) 


—  Upper  daughter-crown 

!  _  Connecting    threads    of    che 

two  crowns  (achromin) 
— ■  Lower  daughter-crown 

Double-star  (amphiaster) 


t  ''/NVr''^ Upper  daughter-nucleus 


Aequatorial    constriction     of 
the  cell-body 

' — -  Lower  daughter-nucleus 


Fig.  ii.— Indirect  Or  mitotic  cell-division  (with  caryolysis  and  caryo- 
kinesis)  from  the  skin  of  the  larva  of  a  salamander.     (From  Rabl.) 

focus  in  a  "  sphere  of  attraction  "  for  the  granules  of  proto- 
plasm in  the  surrounding  cell-body,  and  assumes  a  star-like 
appearance  (the  cell-star,  or  monaster).  The  two  centroso- 
mata,  standing  opposed  to  each  other  at  the  poles  of  the 
nuclear  spindle,  form  "the  double-star"  (or  amphiaster,¥\g.  n, 


THE  OrC.U  AND   THE  AMcE/l.l 


B,  C).  The  chromatin  often  forms  a  long-,  irregularly-wound 
thread — "the  coil"  (spiremay  Fig.  A).  At  the  commence- 
ment of  the  cleavage  it  gathers  at  the  equator  of  the  cell, 
between  the  stellar  poles,  and  forms  a  crown  of  U-shaped 
loops  (generally  four  or  eight,  or  some  other  definite 
number).  The  loops  split  lengthwise  into  two  halves  (B), 
and  these  back  away  from  each  other  towards  the  poles  of 
the  spindle  (C).  Here  each  group  forms  a  crown  once  more, 
and  this,  with  the  corresponding  half  of  the  divided  spindle, 
forms  a  fresh  nucleus  (D).  Then  the  protoplasm  of  the  cell- 
body  begins  to  contract  in  the  middle,  and  gather  about  the 
new  daughter-nuclei,  and  at  last  the  two  daughter-cells 
become  independent  beings. 

Between  this  common  mitosis,  or  indirect  cell-division — 
which  is  the  normal  cleavage-process  in  most  cells  of  the 
higher  animals  and  plants — -and  the  simple  direct  division 
(Fig.  10)  we  find  every  grade  of  segmentation;  in  some 
circumstances  even  one  kind  of  division  may  be  converted 
into  another  (as,  for  instance,  in  the  segmentation  of  the 
yelk-cells  in  discoblastic  ova). 

The  plastid  is  also  endowed  with  the  functions  of  move- 
ment and  sensation.  The  single  cell  can  move  and  creep 
about,  when  it  has  space  for  free  movement  and  is  not 
prevented  by  a  hard  envelope  ;  it  then  thrusts  out  at  its 
surface  processes  like  fingers,  and  quickly  withdraws  again, 
and  thus  changes  its  shape  (Fig.  12).  Finally,  the  young  cell 
is  sensitive,  or  more  or  less  responsive  to  stimuli;  it  makes 
certain  movements  on  the  application  of  chemical  and 
mechanical  irritation.  Hence  we  can  ascribe  to  the  individual 
cell  all  the  chief  functions  which  we  comprehend  under  the 
general  heading  of  "  life  " — sensation,  movement,  nutrition, 
and  reproduction.  All  these  properties  of  the  multicellular 
and  highly  developed  animal  are  also  found  in  the  single 
animal-cell,  at  least  in  its  younger  stages.  There  is  no 
longer  any  doubt  about  this,  and  so  we  may  regard  it  as  a 
solid  and  important  base  of  our  physiological  conception  of 
the  elementary  organism. 

Without  going  any  further  here  into  these  very  interesting 


THE  OVUM  AND  THE  AMCEBA 


phenomena  of  the  life  of  the  cell,  we  will  pass  on  to  consider 
the  application  of  the  cell  theory  to  the  ovum.  Here  com- 
parative research  yields  the  important  result  that  every  ovum 
is  at  first  a  simple  cell.  I  say  this  is  very  important,  because 
our  whole  science  of  ontogeny  now  resolves  itself  into  the 
problem  :  "How  does  the  multicellular  organism  arise  from 
the  unicellular?"  Every  organic  individual  is  at  first  a 
simple  cell,  and  as  such  an  elementary  organism,  or  a  unit  of 
individuality.     This    cell    produces    a    cluster    of    cells    by 

segmentation,  and  from  these 
developes  the  multicellular 
organism,  or  individual  of 
higher  rank. 

When  we  examine  a  little 
closer  the  original  features  of 
the  ovum,  we  notice  the  ex- 
tremely significant  fact  that  in 
its  first  stage  the  ovum  is  just 
the  same  simple  and  indefinite 
structure  in  the  case  of  man 
and  all  the  animals  (Fig.  13). 
We  are  unable  to  detect  any 
material      difference      between 

freely  about,  by  (like  the  amceba  or       h  jth       j  t        gh  Qr 

rhizopods)  protruding  fine  processes  '  r 

the  uncovered  protoplasmic  internal  constitution.  Later, 
though  the  ova  remain  unicel- 
lular, they  differ  in  size  and 
shape,  enclose  various  kinds  of 
yelk-particles,  have  different 
envelopes,  and  so  on.  But 
when  we  examine  them  at  their  birth,  in  the  ovary  of 
the  female  animal,  we  find  them  to  be  always  of  the 
same  form  in  the  first  stages  of  their  life.  In  the  beginning 
each  ovum  is  a  very  simple,  roundish,  naked,  mobile 
cell,  without  a  membrane  ;  it  consists  merely  of  a  particle 
of  cytoplasm  enclosing  a  nucleus  (Fig.  13).  Special  names 
have  been  given  to  these  parts  of  the  ovum  ;  the  cell- 
body  is  called  the  yelk  fvitellusj,  and  the  cell-nucleus  the 


Fig.  12.— Mobile  cells  from  the 
inflamed  eye  of  a  frog  (from  the 

watery  fluid   of  the   eye,  the  humor 
aqueus).       The     naked    cells    creep 


from  the  uncovered  protoplasmic 
bodv.  These  bodies  vary  continually 
in  number,  shape,  and  size.  The 
nucleus  of  these  amoeboid  lymph- 
cells  ("  travelling-  cells,"  or  plano- 
cytes)  is  invisible,  because  concealed 
by  the  numbers  of  fine  granules  which 
are  scattered  in  the  protoplasm. 
(From  Frey.) 


THE  OVCM  AMI  THE  AMCEBA 


germinal  vesicle  r  vesica  la  germinativa  j.  As  a  rule,  the 
nucleus  of  the  ovum  is  soft,  and  like  a  small  pimple  or 
vesicle.  Inside  it,  as  in  many  other  cells,  there  is  a  nuclear 
skeleton  or  frame  and  a  third,  hard  nuclear  body  (the 
nucleolus).  In  the  ovum  this  is  called  the  germinal  spot 
( macula  germinal iva ).     Finally,  we  find  in  many  ova  (but 


Fig.  13.— Ova  of  various  animals,  executing  amoeboid  movements, 
highly  magnified.    All  the  ova  are  naked  cells  of  varying  shape.     In  the  dark 

led  protoplasm  (yelk)  is  a  large  vesicular  nucleus  (the  germinal 
and  m  this  is  seen  a  nuclear  body  (the  germinal  spot),  in  which  again  we  often 
rminal  point.  Figs.  .1/  A4  represent  the  ovum  of  a  sponge  (leuculmis 
echinus)  in  lour  successive  movements.  A'/  B8  are  the  ovum  of  a  parasitic 
crab  (chondracanthiu  cornutus),  in  eight  successive  movements.  (From 
Edward  von  Beneden.)  Ci  Cg  show  the  ovum  of  the  cat  in  various  si 
movement  (from  Pfluger);  Fig.  I)  the  ovum  of  a  trout;  A' the  ovum  of  a 
chicken  ;  Fa  human  ovum. 


THE  OVUM  AND  THE  AMCEBA 


not  in  all)  a  still  further  point  within  the  germinal  spot,  a 
"nucleolin,"  which  goes  by  the  name  of  the  germinal  point 
(punctum  germinativum).  The  latter  parts  (germinal  spot 
and  germinal  point)  have,  apparently,  a  minor  importance,  in 
comparison  with  the  other  two  (the  yelk  and  germinal  vesicle). 
In  the  yelk  we  must  distinguish  the  active  formative  yelk 
(or  protoplasm  =  first  plasm)  from  the  passive  nutritive  yelk 
(or  deutoplasm  =  second  plasm). 


Fig  14. — The  human  OVUm,  taken  from  the  female  ovary,  magnified  500 
times.  The  whole  ovum  is  a  simple  globular  cell.  The  chief  part  of  the 
globular  mass  is  formed  by  the  nuclear  yelk  (deutoplasm),  which  is  easily  dis- 
tributed in  the  active  protoplasm,  and  consists  of  numbers  of  fine  yelk-granules. 
In  the  upperpart  of  the  yelk  is  the  transparent  globular  germinal  vesicle,  which 
corresponds  to  the  nucleus.  This  encloses  a  darker  granule,  the  germinal  spot, 
which  shows  a  nucleolus.  The  globular  yelk  is  surrounded  by  the  thick 
transparent  germinal  membranes  (ovolemma,  or  zona  pellucida).  This  is 
traversed  by  numbers  of  lines  as  fine  as  hairs,  which  are  directed  radially 
towards  the  centre  of  the  ovum.  These  are  called  the  pore-canals ;  it  is 
through  these  that  the  moving  spermatozoa  penetrate  into  the  yelk  at 
impregnation. 

In  many  of  the  lower  animals  (such  as  sponges,  polyps, 
and  medusa?)  the  naked  ova  retain  their  original  simple 
appearance  until  impregnation.  But  in  most  animals  they 
at  once  begin  to  change  ;  the  change  consists  partly  in  the 
formation    of    connections    with    the   yelk,    which    serve    to 


THE  OYIM  AND  THE  AMCEBA 


nourish  the  ovum,  and  partly  of  external  membranes  for  their 
protection  (the  ovolemma,  or  prochorion).  A  membrane  of 
this  sort  is  formed  in  all  the  mammals  in  the  course  of  the 
embryonic  process.  The  little  globule  is  surrounded  by  a 
thick  capsule  of  glass-like  transparency,  the  zona  pellucida, 
or  ovolemma  pellucidum  (Fig.  14).  When  we  examine  it 
closely  under  the  microscope,  we  see  very  line  radial  streaks 
in  it,  piercing  the  zona,  which  are  really  very  narrow 
canals.  The  human  ovum,  whether  fertilised  or  not,  cannot 
be  distinguished  from  that  of  most  of  the  other  mammals.  It 
is  nearly  the  same  everywhere  in  form,  size,  and  composition. 
When  it  is  fully  formed,  it  has  a  diameter  of  (on  an  average) 
about  i'o  of  an  inch.  When  the  mammal  ovum  has  been 
carefully  isolated,  and  held  against  the  light  on  a  glass-plate, 
it  may  be  seen  as  a  fine  point  even  with  the  naked  eye.  The 
ova  of  most  of  the  higher  mammals  are  about  the  same  size. 
The  diameter  of  the  ovum  is  almost  always  between  ■.'.■■■  and  iV 
of  a  line  (0.1 — 0.2  millimetres).  It  has  always  the  same 
globular  shape;  the  same  characteristic  membrane;  the  same 
transparent  germinal  vesicle  with  its  dark  germinal  spot. 
Even  when  we  use  the  most  powerful  microscope  with  its 
highest  power,  we  can  detect  no  material  difference  between 
the  ova  of  man,  the  ape,  the  dog,  and  so  on.  I  do  not  mean 
to  say  that  there  are  no  differences  between  the  ova  of  these 
different  mammals.  On  the  contrary,  we  are  bound  to 
assume  that  there  are  such,  at  least  as  regards  chemical 
composition.  Even  the  ova  of  different  men  must  differ  from 
each  other  ;  otherwise  we  should  not  have  a  different  indivi- 
dual from  each  ovum.  In  accordance  with  the  law  of  the 
unlikeness  of  individuals,  we  must  assume  that  "all  organic 
individuals  differ  from  the  very  beginning  of  their  development, 
though  they  resemble  each  other  so  much  "  (  Gen.  Morph., 
Band  II.,  S  202).  It  is  true  that  our  crude  and  imperfect 
apparatus  cannot  detect  these  subtle  individual  differences, 
which  are  probably  in  the  molecular  structure.  However, 
such  a  striking  morphological  resemblance  of  their  ova,  so 
great  as  to  seem  to  be  a  complete  similarity,  is  a  strong  proof 
of  the  common   parentage  of  man  and  the  other  mammals. 


THE  OVUM  AXD  THE  A  MCE  B A 


From  the  common  germ-form  we  infer  a  common  stem-form. 

On  the  other  hand,  there  are  striking  peculiarities  by  which 

we  can  easily  distinguish  the  fertilised  ovum  of  the  mammal 

from  the  fertilised  ovum  of  the  birds,  amphibia,  fishes,  and 

other  vertebrates  (see  the  close  of  the  twenty-ninth  chapter). 

The  fertilised  bird-ovum  (Fig.  15)  is  notably  different.     It 

is  true  that  in  its  earliest  stage  (Fig.  13  E)  this  ovum  also  is 

very  like  that  of  the  mammal  (Fig.  13  F).     But  afterwards, 

while    still    within    the    oviduct,    it    takes    up    a    quantity    of 

nourishment  and  works  this  into  the  familiar  large  yellow 

yelk.     When  we  examine  a  very  young  ovum   in  the  hen's 

oviduct,   we   find   it   to   be   a   simple, 

small,  naked,  amoeboid  cell,  just  like 

the  young  ova  of  other  animals  (Fig. 

13).     But  it  then  grows  to  the  size  we 

are  familiar  with  in  the  globular  yelk 

of  the  egg.    The  nucleus  of  the  ovum, 

or  the  germinal  vesicle,  is  thus  pressed 

right  to    the  surface   of  the  globular 

ovum,    and    is   embedded   there    in    a 

Fig.    15.— A   fertilised    small  quantity  of  transparent  matter, 
ovum  from  the  oviduct  M  J  F  ' 

of  a  hen.   The  yellow  yelk    the  so-called  white  yelk.     This  forms 

(c)   consists  of  several  con-  ,         ,   .  ,   .    ,      .       , 

round  white  spot,    which  is  known 


as  the  egg-scar  (  cicatricula)  (Fig. 
15  b).  From  the  scar  a  thin  column 
of  the  white  yelk  penetrates  through 


centric  layers  (d),  and  is 
enclosed  in  a  thin  yelk- 
membrane  [a).  The  nucleus 
or  germinal  vesicle  is  seen 
above  in  the  cicatrix  (b). 
From  that  point  the  white 
yelk  penetrates  to  the  cen-      . 

trai  yelk-cavity  {d').    The    the   yellow  yelk   to  the  centre  or   the 
two  kinds  of  yelk  do  not    globular  cell,  where  it   swells  into  a 

diner  very  much.  & 

small,  central  globule  (wrongly  called 
the  yelk-cavity,  or  latebra,  Fig.  15  d).  The  yellow  yelk- 
matter  which  surrounds  this  white  yelk  has  the  appearance 
in  the  egg  (when  boiled  hard)  of  concentric  layers  (c). 
The  yellow  yelk  is  also  enclosed  in  a  delicate  structureless 
membrane  (the  membrana  vitelline!,  a). 

As  the  large  yellow  ovum  of  the  bird  attains  a  diameter  of 
several  inches  in  the  bigger  birds  and  encloses  vesicular  yelk- 
particles,  there  was  formerly  a  reluctance  to  consider  it  as  a 
simple  cell.     This,  however,  was  an  error  from  which  His 


THE  O  1 7  .1/  .  I  \7>  THE  .  \M(EB.  1 


and  other  embrvologists  have  even  recently  drawn  wrong 
conclusions,  though  it  was  corrected  by  Gegenbaur  forty 
years  ago.  The  unfertilised  and  undivided  ovum  of  the  bird 
remains  a  real  cell  with  its  simple  nucleus,  however  large  it 
may  grow  by  the  production  of  yellow  yelk.  Every  animal 
that  lias  only  one  cell-nucleus,  every  amoeba,  every  gregarina, 
every  infusorium,  is  unicellular,  and  remains  unicellular 
whatever  variety  of  matter  it  feeds  on.  So  the  ovum 
remains  a  simple  cell,  however  much  yellow  yelk  it  after- 
wards accumulates  within  its  protoplasm.  Gegenbaur  and 
Van  Beneden  have  clearly  shown  this  in  their  admirable 
works  on  the  ova  of  mammals. 

It  is,  of  course,  different  with  the  bird's  egg  when  it  has 
been  fertilised.  Then  its  nucleus  multiplies  by  repeated 
cleavage,  and  the  protoplasm  of  the  cicatrix  which  surrounds 
it  is  similarly  divided.  The  ovum  then  consists  of  as  many 
cells  as  there  are  nuclei  in  the  cicatrix.  Hence,  in  the 
fertilised  egg  which  we  eat  daily,  the  yellow  yelk  is  already  a 
multicellular  body.  Its  scar  is  composed  of  several  cells,  and 
is  now  commonly  called  the  germinal  disc  (discus  blasto- 
dermicus  >.  We  shall  return  to  this  discogastrida  in  the 
ninth  chapter. 

When  the  mature  bird-ovum  has  left  the  ovary  and  been 
fertilised  in  the  oviduct,  it  covers  itself  with  various  mem- 
branes which  are  secreted  from  the  wall  of  the  oviduct. 
First,  the  large  clear  albuminous  layer  is  deposited  around 
the  yellow  yelk;  afterwards,  the  hard  external  shell,  with  a 
fine  inner  skin.  All  these  gradually  forming  envelopes  and 
processes  are  of  no  importance  in  the  formation  of  the 
embryo;  they  serve  merely  for  the  protection  of  the  original 
simple  ovum.  We  sometimes  find  extraordinarily  large  eggs 
with  strong  envelopes  in  the  case  of  other  animals,  such  as 
fishes  of  the  shark  type.  Hut  here,  also,  the  ovum  is 
originally  of  the  same  character  as  it  is  in  the  mammal  ;  it  is 
a  perfectly  simple  and  naked  cell.  But,  as  in  the  case  of  the 
bird,  a  considerable  quantity  of  nutritive  yelk  is  accumulated 
inside  the  original  yelk  as  food  for  the  developing  embryo  ; 
and  various  coverings  are  formed  round  the  egg.     The  ovum 


THE  OVUM  AND  THE  AMCEBA 


of  many  other  animals  has  the  same  internal  and  external 
features.  They  have,  however,  only  a  physiological,  not  a 
morphological,  importance;  they  have  no  direct  influence  on 
the  formation  of  the  foetus.  They  are  partly  consumed  as 
food  by  the  embryo,  and  partly  serve  as  protective  envelopes. 
Hence  we  may  leave  them  out  of  consideration  altogether 
here,  and  restrict  ourselves  to  material  points — to  the  sub- 
stantial identity  of  the  original  ovum  in  man  and  the  rest  of 
the  animals  (Fig.  13). 

Now,  let  us  for  the  first  time  make  use  of  our  biogenetic 
law,  and  directly  apply  this  fundamental  law  of  evolution  to 
the  human  ovum.  We  reach  a  very  simple,  but  very  impor- 
tant, conclusion.  From  the  fact  that  the  human  ovum  and 
that  of  all  other  animals  consists  of  a  single  cell,  it  follows 
immediately,  according  to  the  biogenetic  laiv,  that  all  the 
animals,  including  man,  descend  from  a  unicellular  organism. 
If  our  biogenetic  law  is  true,  if  the  embryonic  development  is 
a  summary  or  condensed  recapitulation  of  the  stem-history — 
and  there  can  be  no  doubt  about  it — we  are  bound  to  conclude, 
from  the  fact  that  all  the  ova  are  at  first  simple  cells,  that  all 
the  multicellular  organisms  originally  sprang  from  a  unicel- 
lular being.  And  as  the  original  ovum  in  man  and  all  the 
other  animals  has  the  same  simple  and  indefinite  appearance, 
we  may  assume  with  some  probability  that  this  unicellular 
stem-form  was  the  common  ancestor  of  the  whole  animal 
world,  including  man.  However,  this  last  hypothesis  does 
not  seem  to  me  as  inevitable  and  as  absolutely  certain  as  our 
first  conclusion. 

Thisjnference  from  the  unicellular  embryonic  form  to  the 
unicellular  ancestor  is  so  simple,  but  so  important,  that  we 
cannot  sufficiently  emphasise  it.  We  must,  therefore,  turn 
next  to  the  question  whether  there  are  to-day  any  unicellular 
organisms,  from  the  features  of  which  we  may  draw  some 
approximate  conclusion  as  to  the  unicellular  ancestors  of  the 
multicellular  organisms.  The  answer  is  :  Most  certainly 
there  are.  There  are  assuredly  still  unicellular  organisms 
which  are,  in  their  whole  nature,  really  nothing  more  than 
permanent  ova.    There  are  independent  unicellular  organisms 


THE  OIT.]/  AND  THE  AMCEBA 


of  the  simplest  character  which  develop  no  further,  but  repro- 
duce themselves  as  such,  without  any  further  growth.  We 
know  to-day  of  a  great  number  o(  these  little  beings,  such  as 
the  gregarina,  flagellata,  acineta,  infusoria,  etc.  However, 
there  is  one  of  them  that  has  an  especial  interest  for  us, 
because  it  at  once  suggests  itself  when  we  raise  our  question, 
and  it  must  be  regarded  as  the  unicellular  being  that 
approaches  nearest  to  the  real  ancestral  form.  This  organism 
is  the  amoeba. 

For  a  long  time  now  we  have  comprised  under  the  general 

name  of  amoeba?  a   number  of  micro- 
scopic unicellular  organisms,  which  are 

very    widely  distributed,    especially    in 

fresh  water,  but  also  in  the  ocean  ;  in 

fact,  they  have  lately  been  discovered 

in  damp  soil.     There  are  also  parasitic 

amoebae  which  live  inside  other  animals. 

When  we  place  one  of  these  amoeba?  in 

a  drop  of  water  under  the  microscope 

and  examine   it  with  a   high   power,  it 

generally  appears  as  a  roundish  particle 

.                                       F  Fie.   16.  —A  creeping 

ot  a  very  irregular  and  varying  shape  amoeba    (highly    magm- 

/i--             c          ,        ,        ,       •            r       ,  •  fied).    The  whole  organism 

(bigs.    10   and   17).      In   Its  Soft,  slimy,  is  a'simpie  naked  clu,  and 

semi-fluid  substance,  which  consists  of    move?  alHH"  by  meanflf 

the  changing  arms   winch 

protoplasm,    we    see    only    the    solid    it  thrusts  out  of  and  with- 

...  .  .  draws  into  its  protoplasmic 

globular  particle  it  contains,  the  nucleus,    body.     Inside    it    is   the 

Thic      ...-,:,  -.11..  I  u      1  u  roundish    nucleus    with    its 

inis  unicellular  body  moves  about  nucieoiUSl 
continually,  creeping  in  every  direc- 
tion on  the  glass  on  which  we  are  examining  it.  The 
movement  is  effected  by  the  shapeless  body  thrusting  out 
finger-like  processes  at  various  parts  of  its  surface;  and 
these  are  slowly  but  continually  changing,  and  drawing 
the  rest  of  the  body  after  them.  After  a  time,  perhaps, 
the  action  changes.  The  amceba  suddenly  stands  still, 
withdraws  its  projections,  and  assumes  a  globular  shape. 
In  a  little  while,  however,  the  globular  body  begins  to 
expand  again,  thrusts  out  arms  in  another  direction,  and 
moves    on    once    more.     These   changeable    processes    are 


THE  OVUM  AND  THE  AMCEBA 


called  "false  feet,"  or  pseudopodia,  because  they  behave 
physiologically  as  feet,  yet  are  not  special  organs  in  the 
anatomic  sense.  They  disappear  as  quickly  as  they  come, 
and  are  nothing  more  than  temporary  projections  of  the  semi- 
fluid, homogeneous,  and  structureless  body. 

If  you  touch  one  of  these  creeping  amoeba;  with  a  needle, 
or  put  a  drop  of  acid  in  the  water,  the  whole  body  at  once 
contracts  in  consequence  of  this  mechanical  or  physical 
stimulus.  As  a  rule,  the  body  then  resumes  its  globular 
shape.  In  certain  circumstances — for  instance,  if  the  impurity 
of  the  water  lasts  some  time — the  amcebas  begins  to  develop 
a  covering.  It  exudes  a  homogeneous  membrane  or  capsule, 
which  immediately  hardens,  and  assumes  the  appearance  of 
a  globular  cell  with  a  protective  membrane.  The  amoeba 
either  takes  its  food  directly  by  imbibition  of  matter  floating 
in  the  water,  or  by  pressing  into  its  protoplasmic  body  solid 
particles  with  which  it  comes  in  contact.  The  latter  process 
may  be  observed  at  any  moment  by  forcing  it  to  eat.  If 
finely  ground  colouring  matter,  such  as  carmine  or  indigo, 
is  put  into  the  water,  you  can  see  the  soft  body  of  the  amoeba 
pressing  these  coloured  particles  into  itself,  the  substance  of 
the  cell  closing  round  them.  The  amoeba  can  take  in  food 
in  this  way  at  any  point  on  its  surface,  without  having  any 
special  organs  for  intussusception  and  digestion,  or  a  real 
mouth  or  gut. 

The  amoeba  grows  by  thus  taking  in  food  and  dissolving 
the  particles  eaten  in  its  protoplasm.  When  it  reaches  a 
certain  size  by  this  continual  feeding,  it  begins  to  reproduce. 
This  is  done  by  the  simple  process  of  cleavage  (Fig.  17). 
First,  the  nucleus  divides  into  two  parts.  Then  the  proto- 
plasm is  separated  between  the  two  new  nuclei,  and  the  whole 
cell  splits  into  two  daughter-cells,  the  protoplasm  gathering 
about  each  of  the  nuclei.  The  thin  bridge  of  protoplasm 
which  at  first  connects  the  daughter-cells  soon  breaks.  Here 
we  have  the  simple  form  of  direct  cleavage  of  the  nuclei. 
Without  mitosis,  or  formation  of  threads,  the  homogeneous 
nucleus  divides  into  two  halves.  These  move  away  from 
each  other,  and  become  centres  of  attraction  for  the  enveloping 


THE  off.U  AND  THE  AMCEBA 


matter,  the  protoplasm.  The  same  direct  cleavage  o(  the 
nuclei  is  also  witnessed  in  the  reproduction  oi'  many  other 
protists,  while  other  unicellular  organisms  show  the  indirect 
division  of  the  cell. 

Hence,  although  the  amoeba  is  nothing  but  a  simple  cell, 
it  is  evidently  able  to  accomplish  all  the  functions  of  the 
multicellular  organism.  It  moves,  feels,  nourishes  itself,  and 
reproduces.  Some  kinds  of  these  amoebae  can  be  seen  with 
the  naked   eye,  but    most   of  them   are   microscopically  small. 


Fig.  17. —Division  of  a  unicellular  amoeba  (amoeba  polypodia)  in  six 
stages.  (From  F.  E.  Stluiltze.)  The  dark  spot  is  the  nucleus,  the  lighter  spot 
a  contractile  vacuole  in  the  protoplasm.  The  latter  re-forms  in  one  of  the 
daughter-cells. 

It  is  for  the  following  reasons  that  we  regard  the  amoeba;  as 
the  unicellular  organisms  which  have  special  phylogenetic 
(or  evolutionary)  relations  to  the  ovum.  In  many  of  the 
lower  animals  the  ovum  retains  its  original  naked  form  until 
fertilisation,  developes  no  membranes,  and  is  then  often 
indistinguishable  from  the  ordinary  amceba.  Like  the 
amoeba?,  these  naked  ova  may  thrust  out  processes,  and  move 
about  as  travelling  cells.     In  the  sponges  these  mobile  ova 


THE  OVUM  AXD  THE  A  MCE B  A 


move  about  freely  in  the  maternal  body  like  independent 
amoeba;  (Fig.  17).  They  had  been  observed  by  earlier 
scientists,  but  described  as  foreign  bodies — namely,  parasitic 
amoeba;,  living  parasitically  on  the  body  of  the  sponge. 
Later,  however,  it  was  discovered  that  they  were  not  para- 
sites, but  the  ova  of  the  sponge.  We  also  find  this  remark- 
able phenomenon  among  other  animals,  such  as  the  graceful, 
bell-shaped  zoophyta,  which  we  call  polyps  and  medusa;. 
Their  ova  remain  naked  cells,  which  thrust  out  amoeboid 
projections,  nourish  themselves,  and  move  about.  When 
they  have  been  fertilised,  the  multicellular  organism  is  formed 
from  them  by  repeated  segmentation. 

It  is,  therefore,  no  audacious  hypo- 
thesis, but  a  perfectly  sound  conclusion, 
to  regard  the  amoeba  as  the  particular 
unicellular  organism  which  offers  us  an 
approximate  illustration  of  the  ancient 
common  unicellular  ancestor  of  all  the 
metazoa,  or  multicellular  animals.  The 
simple  naked  amoeba  has  a  less  definite 

Fig.  18. -Ovum  of  a  and    more    original    character    than    any 
sponge(o/y>it/ntsj.  The  other  cell.     Moreover,   there   is  the  fact 

ovum    creeps    about    in  ,.  j  i_ 

the  bodv  of  the  sponge  that  recent  research  has  discovered  such 
ftaKJp^Tu  amoeba-like  cells  everywhere  in  the 
is  indistinguishable  from  mature  body  of  the  multicellular  animals. 

the  common  amoeba.  ' 

They  are  found,  for  instance,  in  the 
human  blood,  side  by  side  with  the  red  corpuscles,  as 
colourless  blood-cells ;  and  it  is  the  same  with  all  the  verte- 
brates. They  are  also  found  in  many  of  the  invertebrates — 
for  instance,  in  the  blood  of  the  snail.  I  showed,  in  1859, 
that  these  colourless  blood-cells  can,  like  the  independent 
amoeba;,  take  up  solid  particles,  or  "eat"  (whence  they  are 
called  phagocytes  =  "eating-cells,"  Fig.  19).  Lately,  it  has 
been  discovered  that  many  different  cells  may,  if  they  have 
room  enough,  execute  the  same  movements,  creeping  about 
and  eating.  They  behave  just  like  amoeba;  (Fig.  12).  It  has 
also  been  shown  that  these  "  travelling-cells,"  or  planocy tes, 
play  an   important  part  in   man's  physiology  and   pathology 


THE  OVCM  AND  THE  AMCEBA 


(as   moans  of  transport  for  food,  infectious   matter,    bacteria, 

etc.). 

The  power  of  the  naked  cell  to  execute  these  characteristic 
amoeba-like  movements  comes  from  the  contractility  (or  auto- 
matic mobility)  of  its  protoplasm.  This  seems  to  be  a 
universal  property  of  young  cells.  When  they  are  not 
enclosed  by  a  firm  membrane,  or  confined  in  a  "cellular 
prison,"  they  can  always  accomplish  these  amceboid  move- 
ments. This  is  true  of  the  naked  ova  as  well  as  of  any  other 
naked  cells,  of  the 
"travelling-cells  "  of 
various  kinds  in 
connective  tissue,  ot 
the  mesenchymic 
cells,  lymph -cells, 
mucus-cells,  etc. 

We  have  now, 
by  our  study  of  the 
ovum  and  the  com- 
parison of  it  with 
the  amoeba,  pro- 
vided a  perfectly 
sound  and  most 
valuable  foundation 
for  both  the  embrvo- 
logy  and  the  evolu- 
tion of  man.  We 
nave  learned  that  the  human  ovum  is  a  simple  cell,  that 
this  ovum  is  not  materiallv  different  from  that  of  other 
mammals,  and  that  we  may  conclude  from  it  to  the  existence 
of  a  primitive  unicellular  ancestral  form,  witli  a  substantial 
resemblance  to  the  amoeba. 

The  statement  that  the  earliest  progenitors  of  the  human 
race  were  simple  cells  of  this  kind,  and  led  an  independent 
unicellular  life  like  the  amoeba,  has  not  only  been  ridiculed 
as  the  dream  of  a  natural  philosopher,  hut  also  been  violently 
censured  in  theological  journals  as  "  shameful  and  immoral." 
But,  as  I  observed  in  my  essay  On  (tie  Origin  and  Ancestral 


Fig.  iq.—  Blood-cells  that  eat,  op  phago- 
cytes, from  a  naked  sea-snail  ( thetisj,  greatly 

magnified.  I  was  the  first  to  observe  ill  the  blood- 
cells  of  tliis  snail  tin-  important  fact  that  "the 
blood-cells  of  the  invertebrates  are  unprotected 
pieces  of  plasm,  and  take  in  food,  by  means  of 
their  peculiar  movements,  like  the  amoeba?."  I 
had  (in  Naples,  on  May  ioth,  1859)  injected  into  the 
blood-vessles  of  one  of  these  snails  an  infusion  oi 
water  and  ground  indigo,  and  was  greatly 
astonished  to  find  the  blood-cells  themselves  more 
or  less  filled  with  the  particles  of  indigo  after  a  few 
hours.    After   repeated   injections   I  succeeded   in 

"observing  the  very  entrance  o\~  the  coloured 
particles  in  the  blood-cells,  which  took  place  just  in 
the  same  way  as  with  the  amoeba."  I  have  given 
further  particulars  about  this  in  my  Monograph  oil 
the  Radiolaria. 


THE  OVUM  AXD  THE  A  MCE B  A 


Tree  of  the  Human  Race  in  1870,  this  offended  piety  must 
equally  protest  against  the  "  shameful  and  immoral  "  fact  that 
each  human  individual  is  developed  from  a  simple  ovum,  and 
that  this  human  ovum  is  indistinguishable  from  those  of  the 
other  mammals,  and  in  its  earliest  stage  is  like  a  naked 
amoeba.  We  can  show  this  to  be  a  fact  any  day  with  the 
microscope,  and  it  is  little  use  to  close  one's  eyes  to 
"  immoral  "  facts  of  this  kind.  It  is  as  indisputable  as  the 
momentous  conclusions  we  draw  from  it  and  as  the  vertebrate 
character  of  man  (see  Chapter  XL). 

We  now  see  very  clearly  how  extremely  important  the 
cell  theory  has  been  for  our  whole  conception  of  organic 
nature.  "  Man's  place  in  nature  "  is  settled  beyond  question 
by  it.  Apart  from  the  cell  theory,  man  is  an  insoluble  enigma 
to  us.  Hence  philosophers,  and  especially  physiologists, 
should  be  thoroughly  conversant  with  it.  The  soul  of  man 
can  only  be  really  understood  in  the  light  of  the  cell-soul,  and 
we  have  the  simplest  form  of  this  in  the  amoeba.  Only  those 
who  are  acquainted  with  the  simple  psychic  functions  of  the 
unicellular  organisms  and  their  gradual  evolution  in  the 
series  of  lower  animals  can  understand  how  the  elaborate 
mind  of  the  higher  vertebrates,  and  especially  of  man,  was 
gradually  evolved  from  them.  The  academic  psychologists 
who  lack  this  zoological  equipment  are  unable  to  do  so. 

This  naturalistic  and  realistic  conception  is  a  stumbling- 
block  to  our  modern  idealistic  metaphysicians  and  their 
theological  colleagues.  Fenced  about  with  their  transcendental 
and  dualistic  prejudices,  they  attack  not  only  the  monistic 
system  we  establish  on  our  scientific  knowledge,  but  even  the 
plainest  facts  which  go  to  form  its  foundation.  An  instructive 
instance  of  this  was  seen  three  years  ago,  in  the  academic 
discourse  delivered  by  a  distinguished  theologian,  Willibald 
Beyschlag,  at  Halle,  January  12th,  1900,  on  the  occasion  of 
the  centenary  festival.  The  theologian  protested  violently 
against  the  "  materialistic  dustmen  of  the  scientific  world  who 
offer  our  people  the  diploma  of  a  descent  from  the  ape,  and 
would  prove  to  them  that  the  genius  of  a  Shakespeare  or  a 
Goethe    is   merely   a   distillation  from    a   drop   of    primitive 


THE  OVCM  AND   THE  AMCEBA 


mucus."  Another  well-known  theologian  protested  against 
the  horrible  idea  that  the  greatest  of  men,  Luther  and 
Christ,  were  descended  from  a  mere  globule  of  protoplasm." 
Nevertheless,  not  a  single  informed  and  impartial  scientist 
doubts  the  fact  that  these  greatest  men  were,  like  all  other 
men — and  all  other  vertebrates — developed  from  an  impreg- 
nated ovum,  and  that  this  simple  nucleated  globule  of 
protoplasm  has  the  same  chemical  constitution  in  all  the 
mammals. 

The  actual  amoeba?  and  other  unicellular  organisms 
(arcella,  radiolaria,  etc.)  are  of  great  importance  for  our 
conclusion,  because  they  exhibit  these  single  cells  to  us  in 
permanent  independence,  as  autonomous  cells.  The  human 
organism  and  that  of  the  other  higher  animals  are  only  one- 
celled  in  the  earliest  stage  of  existence.  As  soon  as  the 
ovum  is  fertilised,  it  increases  by  segmentation,  and  forms 
a  group  or  colony  of  social  cells,  a  cell-community  or  a 
ccenobium.  These  take  on  different  forms,  and,  by  a  division 
of  labour  among  the  cells  and  their  development  along 
different  lines,  the  multifarious  tissues  that  make  up  the 
animal  body  are  produced.  Thus  the  mature  multicellular 
organism  of  man  and  the  other  higher  animals  and  plants  is 
a  his  ton  (or  "  tissue-body  "),  a  social  community  of  the  various 
kinds  of  tissue-cells.  The  innumerable  organic  units  in  this 
"  histon  "  may  vary  considerably  when  their  development  is 
complete,  but  they  were  originally  simple  cells  of  the  same 
type,  the  equal  citizens  of  the  cell-state. 


CHAPTER  VII. 

CONCEPTION 

The  meaning-  of  sexual  reproduction.  Nature  of  conception  ;  fusion  of  the 
female  ovum  and  male  spermatozoon.  Various  forms  of  the  sperm-cells 
(usually  cone-shaped  ciliary  cells).  Theory  of  the  spermatozoa.  Inheri- 
tance from  both  parent-cells.  The  new  stem-cell  or  cytula.  Its  herma- 
phroditic character.  Process  of  fertilisation  of  ovum :  release  of  the 
germinal  vesicle  and  protrusions  of  the  directing-  body.  Penetration  of 
a  spermatozoon  in  the  body  of  the  ovum  :  movement  and  blending  of  the 
two  pronuclei.  Formation  of  the  stem-nucleus  ( archicaryon ),  the  vehicle  of 
ir'ieritance.     Older  theories  of  conception.       Importance  and  equal  share 

the    two    sexual    cells.     Male    microspores   and    female   macrospores. 

yspermism   of    the    chloroformed   ovum.      Importance   of    this   fact    in 

chology,  the  theory  of  the  cell-soul  and  personal  immortality.     Imperma- 

ice  of  all  that  is  personal  and  individual. 

Th:  recognition  of  the  fact  that  every  man  begins  his 
individual  existence  as  a  simple  cell  is  the  solid  foundation  of 
all  research  into  the  genesis  of  man.  From  this  fact  we  are 
forced,  in  virtue  of  our  biogenetic  law,  to  draw  the  weighty 
phylogenetic  conclusion  that  the  earliest  ancestors  of  the 
human  race  were  also  unicellular  organisms  ;  and  among 
these  protozoa  we  may  single  out  the  vague  form  of  the 
amoeba  as  particularly  important  (cf.  Chapter  VI.).  That  these 
unicellular  ancestral  forms  did  once  exist  follows  directly 
from  the  phenomena  which  we  perceive  every  day  in  the 
fertilised  ovum.  The  development  of  the  multicellular 
organism  from  the  ovum,  and  the  formation  of  the  germinal 
layers  and  the  tissues,  follow  the  same  laws  in  man  and  all 
the  higher  animals.  It  will,  therefore,  be  our  next  task  to 
consider  more  closely  the  impregnated  ovum  and  the  process 
of  conception  which  produces  it. 

The  process  of  impregnation  or  sexual  conception  is  one  of 
those  phenomena  that  people  love  to  conceal  behind  the 
mystic  veil  of  supernatural  power.  We  shall  soon  see, 
however,  that  it  is  a  purely  mechanical  process,  and  can  be 
reduced  to  familiar  physiological  functions.  Moreover,  this 
amphigony  (or  conception)  is  of  the  same  type,  and  is  effected 
124 


C  OXt  F.l'TlOX  1 25 

bv  the  same  organs,  in  man  as  in  all  the  other  mammals. 
The  pairing  of  the  male  and  female  has  in  both  cases  for  its 
main  purpose  the  introduction  of  the  ripe  matter  o(  the  male 
seed  or  sperm  into  the  female  body,  in  the  sexual  canals  of 
which  it  encounters  the  ovum.  Conception  then  ensues  by 
the  blending  of  the  two. 

We  must  observe,  first,  that  this  important  process  is  by 
no  moans  so  widely  distributed  in  the  animal  and  plant  world 
as  is  commonly  supposed.  There  is  a  very  large  number 
of  lower  organisms  which  propagate  unsexually,  or  by 
monogony,  and  especially  the  sexless  monera  (chromacea, 
bacteria,  etc),  but  also  many  other  protists,  such  as  the 
amoeba;,  foraminifera,  radiolaria,  myxomycetae,  etc.  In 
these  there  is  no  fertilisation  whatever;  the  multiplicatir  F 
individuals  and  propagation  of  the  species  take  plae<  y 
unsexual  reproduction,  which  takes  the  form  of  cleav  ;e, 
budding,  or  spore-formation.  The  copulation  of  two  coal  sc- 
ing  cells,  which  in  these  cases  often  precedes  the  reproduc- 
tion, cannot  be  regarded  as  a  sexual  act  when  the  two 
copulating  plastids  differ  in  size  or  structure  (microspores 
and  macrospores).  On  the  other  hand,  sexual  reproduction 
is  the  general  rule  with  all  the  higher  organisms,  both 
animal  and  plant  ;  very  rarely  do  we  find  asexual  reproduc- 
tion among  them.  There  are,  in  particular,  no  cases  of 
parthenogenesis  (virginal  conception)  among  the  vertebrates. 

Sexual  reproduction  offers  an  infinite  variety  of  interesting 
forms  in  the  different  classes  of  animals  and  plants,  especiallv 
as  regards  the  mode  of  conception,  and  the  conveyance  of 
the  spermatozoon  to  the  ovum.  These  features  are  of  great 
importance  not  only  as  regards  conception  itself,  but  for  the 
development  of  the  organic  form  and  especially  for  the  differ- 
entiation of  the  sexes.  There  is  a  particularly  curious  cor- 
relation of  plants  and  animals  in  this  respect.  The  splendid 
studies  o(  Charles  Darwin  and  Hermann  Miiller  on  the  fertili- 
sation of  flowers  by  insects  have  given  us  very  interesting 
particulars  of  this.1     This  reciprocal  service  has  given  rise  to 

'  See   Darwin's  work.  On  t/if  Various  Contrivances  by  which   Orchids  are 
Fertilised  (1862). 


CONCEPTION 


a  most  intricate  sexual  apparatus.  Equally  elaborate  struc- 
tures have  been  developed  in  man  and  the  higher  animals, 
serving  partly  for  the  isolation  of  the  sexual  products  on 
each  side,  partly  for  bringing  them  together  in  conception. 
But,  however  interesting  these  phenomena  are  in  themselves, 
we  cannot  go  into  them  here,  as  they  have  only  a  minor 
importance — if  any  at  all — in  the  real  process  of  conception. 
We  must,  however,  try  to  get  a  very  clear  idea  of  this  pro- 
cess and  the  meaning  of  sexual  reproduction. 

In  every  act  of  conception  we  have,  as  I  said,  to  consider 
two  different  kinds  of  cells — a  female  and  a  male  cell.  The 
female  cell  of  the  animal  organism  is  always  called  the  ovum 
(or  ovulum,  egg,  or  egg-cell) ;  the  male  cells  are  known  as 
the  sperm  or  seed-cells,  or  the  spermatozoa  (also  spermium 
and  zoospermium).  The  female  ovum,  the  form  and  com- 
position of  which  we  have  already  considered,  is  of  the  same 
simple  nature  in  the  early  stages  in  all  the  animals.  It  is  at 
first  merely  a  globular  naked  cell,  consisting  of  protoplasm 
and  a  nucleus  (Fig.  13).  When  it  has  freedom  to  move,  it 
often  makes  slow  amoeboid  movements,  as  we  have  seen  in 
the  case  of  the  ovum  of  the  sponge  (Fig.  18).  But,  as  a  rule, 
it  is  enclosed  subsequently  by  a  number  of  very  different,  and 
often  very  complicated,  shells  or  membranes.  The  ripe  ovum 
is,  on  the  whole,  one  of  the  largest  cells  we  know.  It  attains 
colossal  dimensions  when  it  absorbs  great  quantities  of 
nutritive  yelk,  as  is  the  case  with  birds  and  reptiles,  and 
many  of  the  fishes.  In  the  great  majority  of  the  animals 
the  ripe  ovum  is  rich  in  yelk  and  much  larger  than  the 
other  cells. 

On  the  other  hand,  the  next  cell  which  we  have  to  con- 
sider in  the  process  of  conception,  the  male  sperm-cell  or 
spermatazoon,  is  one  of  the  smallest  cells  in  the  animal  body. 
Conception  usually  consists  in  the  bringing  into  contact  with 
the  ovum  of  a  slimy  fluid  secreted  by  the  male,  and  this  may 
take  place  either  inside  or  out  of  the  female  body.  This  fluid 
is  called  sperm,  or  the  male  seed.  Sperm,  like  saliva  or 
blood,  is  not  a  simple  fluid,  but  a  thick  agglomeration  of 
innumerable  cells,  swimming  about  in  a  comparatively  small 


CO.XCEPTIOX 


quantity   of    fluid.      It   is   not  the  fluid,   but  the   independent 
male  cells  that  swim  in  it,  that  cause  conception. 

The  spermatozoa  of  the  great  majority  of  animals  have 
two  characteristic  features.  Firstly,  they  are  extraordinarily 
small,  being  usually  the  smallest  cells  in  the  body  ;  and, 
secondly,  they  have,  as  a  rule,  a  peculiarly  lively  motion, 
which  is  known  as  spermatozoic  motion.  The  shape  of  the 
cell  has  a  good  deal  to  do  with  this  motion.  In  most  of  the 
animals,  and  also  in  many  of  the  lower  plants  (but  not  the 
higher),  each  of  these  spermatozoa  has  a  very  small,  naked 


Fig.  20.-  Spermia  or  spermatozoa  from  the  male  sperm  of  various 
mammals.  The  pear-shaped  flattened  nucleus  of  the  seed-cell  (the  so-called 
•■  head  of  the  spermatozoon  ")  is  seen  from  the  front  in  /. ,  and  sideways  in  //. 
k  i-.  the  nucleus,  "/  its  middle  pari  (protoplasm),  s  the  mobile,  serpent-like  tail 
(or  whip)  ;  .1/  four  human  spermatozoa,  .i  four  spermatozoa  from  the  ape; 
A'  from  the  hare  ;  H  from  the  house-mouse  ;  C  from  the  dog- ;  S  from  the  pig. 

cell-body,  enclosing  an  elongated  nucleus,  and  a  long  thread 
hanging  from  it  (Fig.  20).  It  was  long  before  we  could 
recognise  that  this  structure  is  a  simple  cell.  Thev  were 
formerly  held  to  be  special  organisms,  and  were  called  "seed- 
animals"  (spermato-zoa,  or  spermato-zoidia) ;  they  are  now 
scientifically  known  as  spermia  or  spermidia,  or  as  sperma- 
tosomata  (seed-bodies)  or  spermatofila  (seed  threads).  It  took 
a  good  deal  of  comparative  research  to  convince  us  that  each 
of  these  spermatozoa  is  really  a  simple  cell.  They  have  the 
same  shape  as  in  many  other  vertebrates  and  most  of  the 
invertebrates.      However,  in  many  of  the  lower  animals  they 


CO.XCEPTIOX 


have  quite  a  different  shape.  Thus,  for  instance,  in  the  river 
crab  thev  are  large  round  cells,  without  any  movement, 
equipped  with  stiff  outgrowths  like  bristles  (Fig.  21/).  They 
have  also  a  peculiar  form  in  some  of  the  worms,  such  as  the 
thread-worms  (filarial;  in  this  case,  they  are  sometimes 
amoeboid  and  like  very  small  ova  (Fig.  21  c-e).  But  in 
most  of  the  lower  animals  (such  as  the  sponges  and  polyps) 
they  have  the  same  pine-cone  shape  as  in  man  and  the  other 
mammals  (Fig.  21  a,  h). 

When  the  Dutch  naturalist  Leeuwenhoek  discovered  these 
thread-like  lively  particles  in  1677  in  the  male  sperm,  it  was 
generally  believed  that  they  were 
special,  independent,  tiny  animal- 
cules, like  the  infusoria,  and  so 
were  called  "  seed-animals  "  or 
spermatozoa.  I  have  already 
mentioned  that  they  played  an 
important  part  in  the  pre-forma- 
tion  theory,  as  it  was  believed 
that  the  whole  mature  organism 
existed  already,  with  all  its  parts, 

Fig.    21.  -Spermatozoa    or    but      veiT      sma11     and     packed 
spermidia  of  various  animals,    together,    in    each    spermatozoon 

(rrom  Lang.)     a   oi  a  fish,  b  of  a  °  ^ 

turbeiiaria  (with  two  side-lashes),  (see  p.  27).  The  spermatozoa  had 
c-e  of  a  nematode  (amoeboid  sper-  ,  ,        r      ., 

matozoa),  f  from  a  river   crab    only  to  penetrate  into  the  fertile 

mSldt^wil'h  i\,nHT,-the  sa'a"    soil  of  the  female  ovum,  and  then 

mancler    (with     undulating;   mem-  ' 

brane),  //  of  a  ring-worm  ('«  and  /;  the  pre-formed  body  would  ex- 
are  the  usual  shape). 

pand  and  grow  in  all  its  parts. 
This  erroneous  view  is  now  wholly  abandoned ;  we  know  by 
the  most  accurate  investigation  that  the  mobile  spermatozoa 
are  nothing  but  simple  and  real  cells,  of  the  kind  that  we  call 
"  ciliated  "  (equipped  with  lashes,  or  cilia).  In  the  previous 
illustrations  we  have  distinguished  in  the  spermatozoon  a 
head,  trunk,  and  tail.  The  "  head  "  (Fig.  20  k)  is  merely  the 
oval  nucleus  of  the  cell;  the  body  or  middle-part  (m)  is  an 
accumulation  of  cell-matter ;  and  the  tail  (s)  is  a  thread-like 
prolongation  of  the  same. 

Moreover,  we  now  know  that  these  spermatozoa  are  not  at 


CONCEPTION 


all  a  peculiar  form  of  cell  ;  precisely  similar  cells  are  found  in 
various  other  parts  of  the  body.  If  they  have  many  short 
threads  projecting,  they  are  called  ciliated;  if  only  one  long, 
whip-shaped  process  (or,  more  rarely,  two  or  four),  caudate 
(tailed)  cells.  Caudate  cells,  like  those  of  the  spermatozoa,  are 
found  in  the  gastric  cells  of  the  sponges  and  the  cnidaria. 

Very  careful  recent  examination  of  the  spermia, 
under  a  very  high  microscopic  power  (Fig.  22  a,  b), 
has  detected  some  further  details  in  the  liner 
structure  of  the  ciliated  cell,  and  these  are  common 
to  man  and  the  anthropoid  ape.  The  head  (k) 
encloses  the  elliptic  nucleus  in  a  thin  envelope  of 
cytoplasm;  it  is  a  little  flattened  on  one  side,  and 
thus  looks  rather  pear-shaped  from  the  front  (b). 
In  the  central  piece  (w)  we  can  distinguish  a  short 
neck  and  a  longer  connective  piece  (with  centro- 
soma).  The  tail  consists  of  a  long  main  section 
(//)  and  a  short,  very  fine  tail  (e). 

The  process  of  fertilisation  by  sexual 
conception  consists,  therefore,  essentially  in 
the  coalescence  and  blending  together  of 
two  different  cells.  The  most  curious 
opinions  prevailed  about  this  act  formerly. 
People  always  saw  something  mystic  about 
it,  and  framed  the  most  marvellous  hypo- 
theses on  it.  It  is  only  in  the  last  ten  years 
that    we  have    learned  that    the  process  of 

conception    is   reallv  very  simple  and  has     sicl°-    *  head  (with 

nucleus),   m  middle- 
no  element  of  the  mvsterious.     The  essence     stem,  h    long-stem, 
r  -t   ■      ..     .  ,  ,  •  and   r    tail.      (From 

ot  it  is  that  a  male  spermatozoon  combines     Retsius.) 

with  a  female  ovum.     The  lively  sperma- 
tozoon travels  towards  the  ovum  by  its  serpentine  movements, 
and  bores  its  way  into  the  female  cell  (Fig.  23).      The  nuclei 
ot  both  sexual  cells,  attracted  by  a  certain  "affinity," approach 
each  other  and  melt  into  one. 

This  would  be  an  admirable  place  for  poetic  description  in 
the  most  glowing  colours  of  the  wonderful  mystery  of  concep- 
tion and  the  struggle  of  the  living  spermatozoa,  which  hover 

K 


Fig.  22.— A  single 
human  spermato- 
zoon magnified  2,000 
times  :     a     shows     it 

from  the  broader  and 
b  from  the  narrower 


CONCEPTION 


anxiously  about  the  ovum,  seeking  to  penetrate  nto  the  fine 
porous  canals  of  the  ovolemma  and  plunge  "  consciously  " 
into  the  protoplasmic  yelk,  where  they  die  away  to  find  their 
higher  selves.  The  supporters  of  teleology,  too,  might  pause 
here  to  admire  the  wisdom  of  the  Creator  in  providing  these 
porous  canals  in  the  membrane  of  the  ovum  for  the  sperma- 
tozoa to  enter  through.  However,  the  scientist  coldly 
describes  this  process — this  "  crowning  of  love  " — as  a  blend- 
ing of  two  cells  and  the  combination  of  their  nuclei.  The 
new  cell  that  arises  from  the  process  is  the  simple  product  of 
the  copulation  of  the  two  blending  sexual  cells. 

Hence  the  fertilised  cell  is 
quite  another  thing  from  the  un- 
fertilised cell.  For  if  we  must 
regard  the  spermia  as  real  cells 
no  less  than  the  ova,  and  the 
process  of  conception  £s  a  coa- 
lescence of  the  two,  we  must 
consider  the  resultant  cell  as 
a  quite  new  and  independent 
organism.      It  bears  in  the  cell 

Fig.  23.— The  fertilisation  of    and  nuclear  matter  of  the  pene- 
the  ovum  by  the  spermatozoon  , 

(of  a  mammal).     Oik-  of  the  many  tratlllg    SpermatOZOOn     a    part    OI 

thread-like,  lively  spermidia  pierces  ,        father's     hnrivr      and      in     the 

through  a  fine  pore-canal  into  the  tlle    'atlier  s     DOdy,     ana     in     tne 

nuclear  yelk.    The  nucleus  of  the  protoplasm    and    caryoplasm    of 

ovum  is  invisible. 

the  ovum  a  part  of  the  mother  s 
body.  This  is  clear  from  the  fact  that  the  child  inherits 
many  features  from  both  parents.  It  inherits  from  the  father 
by  means  of  the  spermatozoon  and  from  the  mother  by  means 
of  the  ovum.  The  actual  blending  of  the  two  cells  produces 
a  third  cell,  which  is  the  germ  of  the  child,  or  the  new 
organism  conceived.  One  may  also  say  of  this  sexual 
■coalescence  that  the  stem-cell  is  a  simple  hermaphrodite ;  it 
unites  both  sexual  substances  in  itself. 

I  think  it  necessary  to  emphasise  the  fundamental  impor- 
tance of  this  simple,  but  often  unappreciated,  feature  in  order 
to  have  a  correct  and  clear  idea  of  conception.  With  that 
end,  I  have  given  a  special  name  to  the  new  cell  from  which 


CONCEPTION 


the  child  developes,  and  which  is  generally  loosely  called 
" the  fertilised  ovum  "  or  "  the  first  segmentation  sphere."  I 
call  ii  "the  stem-cell"  (cytula  or  archicytos ,,  its  cell-matter 
"the   stem-plasm"  (archiplasma  or  cytuloplasma ) ,   and    its 

nucleus   "the  stem-nucleus"  (  archicaryon  or  cytulocaryonj. 

The  name  "  stem-cell  "  seems  to  me  the  simplest  and  most 
suitable  because  all  the  other  cells  o(  the  body  are  derived 
from  it,  and  because  it  is,  in  the  strictest  sense,  the  stem- 
father  and  stem-mother  ol~  all  the  countless  generations  of 
cells  o(  which  the  multicellular  organism  is  to  be  composed. 
That  complicated  molecular  movement  o(  the  protoplasm 
which  we  call  "life"  is,  naturally,  something  quite  different 
in  this  stem-cell  from  what  we  find  in  the  two  parent-cells, 
from  the  coalescence  o\  which  it  has  issued.  The  life  of  the 
stem-cell  or  cytula  is  the  product  or  resultant  of  the  paternal 
life-movement  that  is  conveyed  in  the  spermatozoon  and  the 
maternal  life-movement  that  is  contributed  by  the  ovum.  On 
the  principle  of  the  parallelogram  o\  forces,  it  may  be  said 
that  the  potential  energy  of  the  stem-cell  is  the  diagonal  of 
the  parallelogram,  while  its  two  sides  represent  the  potential 
energy  ot  the  paternal  spermatozoa  and  that  of  the  maternal 
bvum.  The  combined  potential  energy  of  the  two,  or  the 
hereditary  potentiality,  is  converted  into  living  force  as  soon 
as  the  individual  development  of  the  stem-cell  begins  after  the 
coalescence. 

The  admirable  work  done  by  recent  observers  has  shown 
that  the  individual  development,  in  man  and  the  other 
animals,  commences  with  the  formation  of  a  simple  "  stem- 
cell  "  k^'  this  character,  and  that  this  then  passes,  by  repeated 
segmentation  (or  fission),  into  a  cluster  oi  cells,  known 
as  "  the  segmentation  sphere "  or  "  segmentation  cells" 
(segmentella  or  blastomeraj.  L'ntil  1875  there  was  a 
spirited  controversy  as  to  the  origin  of  the  stem-cell,  and 
as  to  the  real  behaviour  oi  the  spermatozoon  and  the 
ovum  in  its  formation  or  at  conception.  It  had  been 
generally  assumed  that  the  original  nucleus  o(  the  ovum, 
called  the  germinal  vesicle,  remained  unchanged  at  concep- 
tion, and   passed  over  directly  to  the  stem-nucleus  (or  nucleus 


COXCEPTIOX 


of  "the  first  segmentation  sphere").  However,  most  modern 
observers  are  convinced  that  the  germinal  vesicle  sooner  or 
later  disappears,  and  that  the  stem-nucleus  is  a  new  forma- 
tion. But  there  were  different  opinions  as  to  the  mode  of 
formation  of  this  new  nucleus  of  the  stem-cell.  Some 
thought  that  the  germinal  vesicle  disappeared  before  impreg- 
nation and  some  after.  Some  said  that  it  was  thrust  out 
of  the  ovum,  and  others  that  it  melted  away  in  the  velk. 
Some  believed  that  it  was  wholly,  and  others  that  it  was  only 
partially,  lost.  All  these  contradictory  opinions  and  diffi- 
culties about  these  important  processes  have  now  been 
happilv  settled.  The  solution  began  in  1875,  when  a  number 
of  very  careful  microscopic  studies  of  them  were  published 
about  the  same  time,  especially  those  of  Oscar  Hertwig  and 
Edward  Strasburger  (both  then  at  Jena),  Edward  Van 
Beneden,  O.  Biitschli,  etc.  By  the  work  of  these  many 
succeeding  observers  we  have  gradually  come  to  a  happy 
agreement  as  to  the  essential  features  of  conception,  and  are 
convinced  that  it  has  the  same  physiological  features  in  the 
whole  animal  and  plant  worlds.  This  is  most  clearly 
observed  in  the  ova  of  the  echinoderma  (star-fishes,  sea 
urchins,  sea-gherkins,  etc.).  The  investigations  of  Oscar 
and  Richard  Hertwig  were  chiefly  directed  to  these.  The 
main  results  may  be  summed  up  as  follows  : — 

Conception  is  preceded  by  certain  preliminary  changes, 
which  are  very  necessary — in  fact,  usually  indispensable — for 
its  occurrence.  They  are  comprised  under  the  general 
heading  of  "Changes  prior  to  impregnation."  In  these 
the  original  nucleus  of  the  ovum,  the  germinal  vesicle,  is  lost. 
Part  of  it  is  extruded,  and  part  dissolved  in  the  cell  contents ; 
only  a  very  small  part  of  it  is  left  to  form  the  basis  of  a  fresh 
nucleus,  the  pronucleus  femininus.  It  is  the  latter  alone  that 
combines  in  conception  with  the  invading  nucleus  of  the 
fertilising  spermatozoon  (the pronucleus  masculinusj. 

The  impregnation  of  the  ovum  commences  with  a  decay 
of  the  germinal  vesicle,  or  the  original  nucleus  of  the  ovum 
(Fig.  24).  We  have  seen  that  this  is  in  most  unripe  ova  a 
large,  transparent,   globular  vesicle.     This  germinal  vesicle 


CONCEPTION 


LIBRARY. 


contains  a  viscous  fluid  (the  caryofympm).  j^ffie)  I'M1)  »ihoM^jJuIJ.E 
frame  (caryobasis)   is  formed  oi   the  enveloping  membrane 
and   a   mesh-work   of    nuclear    threads    running   across    the 

interior,  which  is  filled  with  the  nuclear  sap.  In  a  knot  o\ 
the  network  is  contained  the  dark,  stiff,  opaque  nuclear 
corpuscle  or  nucleolus.  When  the  impregnation  of  the  ovum 
sets  in,  (he  greater  part  of  the  germinal  vesicle  is  dissolved  in 

the  cell;  the  nuclear  membrane  and  mesh-work  disappear; 
tile  nuclear  sap  is  distributed  in  the  protoplasm  ;  a  small 
portion  of  the  nuclear  base  is  extruded  ;  another  small 
portion  is  left,  and  is  converted  into  the  secondary  nucleus,  or 
the  female  pro-nucleus  (Fig.  25  rk). 


Vic.  24.  An  unfertilised  ovum  of  an  echinoderm,  with  nuclear  net- 
work and  dark  nucleolus  in  the  large  globular  germinal  vesiole.    |  From  Herttrig. ) 

Fig.  J5.  — An  impregnated  echinoderm  ovum,  with  small  homogeneous 
nucleus  («  i).     1  From  Hertmig.  < 

The  small  portion  of  the  nuclear  base  which  is  extruded 
from    the    impregnated    ovum    is    known   as   the    "  directive 

bodies"  or  "polar  cells";  there  are  many  disputes  as  to 
their  origin  and  significance,  but  we  are  as  yet  imperfectl) 
acquainted  with  them.  As  a  rule,  they  are  two  small  round 
granules,  o(  the  same  size  and  appearance  as  the  remaining 
pro-nucleus.  The  polar  cells  arise  successively  by  the  con- 
striction or  cleavage  of  that  part  oi  the  nuclear  base  (probably, 
as  a  rule,  the  germinal  spot)  which  also  forms  the  female 
pro-nucleus.  We  may,  therefore,  regard  this  cleavage- 
process,  in  which  the  surrounding  protoplasm  shares,  as  a 
twice-repeated  cell  division,  or,  rather,  as  a  gemmation 
(budding)   of  cells ;   because    the    two    parts    into    which    the 


COXCEPTIOX 


impregnated  ovum  divides  each  time  are  not  of  the  same  size 
and  appearance.  The  two  small  polar  cells  are  detached  cell- 
buds;  their  separation  from  the  large  mother-cell  takes  place 
in  the  same  way  as  in  ordinary  "  indirect  cell-division,"  with 
the  formation  of  nuclear  spindle,  plasma  stars,  polar  radia- 
tion, halving  of  the  nuclear  spindle,  mitosis,  etc.  Hence, 
the  polar  cells  are  probably  to  be  conceived  as  "  abortive  ova," 
or  "  rudimentary  ova,"  which  proceed  from  a  simple  original 
ovum  by  cleavage  in  the  same  way  that  several  sperm-cells 
arise  from  one  spermatoblast,  or  one  "sperm-mother-cell,"  in 
spermatogenesis.  The  male  sperm-cells  in  the  testicles  must 
undergo  similar  changes  in  view  of  the  coming  impregnation 
as  the  ova  in  the  female  ovary.  In  this  maturing  of  the 
sperm  each  of  the  original  seed-cells  (spermatoblasts  or 
spermatogonia  J  divides  by  double  segmentation  into  four 
daughter-cells,  each  furnished  with  a  fourth  of  the  original 
nuclear  matter  (the  hereditary  chromatin);  and  each  of  these 
four  descendant  cells  becomes  a  spermium  or  spermatozoon, 
ready  for  impregnation.  Thus  is  prevented  the  doubling  of 
the  chromosomata  and  the  hereditative  chromatin  in  the  coales- 
cence of  the  two  nuclei  at  conception.  As  the  two  polar  cells 
are  extruded  and  lost,  and  have  no  further  part  in  the  fertili- 
sation of  the  ovum,  we  need  not  discuss  them  any  further. 
But  we  must  give  more  attention  to  the  female  pro-nucleus 
which  alone  remains  after  the  extrusion  of  the  polar  cells  and 
the  dissolving  of  the  germinal  vesicle  (Fig.  23  ek).  This 
tiny  round  corpuscle  of  chromatin  now  acts  as  a  centre  of 
attraction  for  the  invading  spermatozoon  in  the  large  ripe 
ovum,  and  coalesces  with  its  "  head,"  the  male  pro-nucleus. 
The  product  of  this  blending,  which  is  the  most  important 
part  of  the  act  of  impregnation,  is  the  stem-nucleus,  or  the 
first  segmentation  nucleus  (  arcliicaryon  1 — that  is  to  say,  the 
nucleus  of  the  new-born  embryonic  stem-cell  or  "first 
segmentation  cell  "  (archicytos  or  cytulaj.  This  stem-cell  is 
the  starting-point  of  the  subsequent  embryonic  processes. 

Hertwig  has  shown  that  the  tiny  transparent  ova  of  the 
echinoderms  are  the  most  convenient  for  following  the  details 
of  this  important  process  of  impregnation.      We  can,  in  this 


CONCEPTION 


case,  easily  and  successfully  accomplish  artificial  impregna- 
tion, and  follow  the  formation  o(  the  stem-cell  step  by  step 
within  the  space  k^\  ten  minutes.  If  we  put  ripe  ova  o(  the 
star-fish  or  sea-urchin  in  a  watch-glass  with  sea-water  and  add 
a  drop  o(  ripe  sperm-fluid,  we  find  each  ovum  impregnated 
within  five  minutes.  Thousands  of  the  fine,  mobile  ciliated 
cells,  which  we  have  described  as  "  sperm-threads"  (Fig.  20), 
make  their  way  to  the  ova,  owing  to  a  sort  o(  chemical 
sensitive  action  which  may  he  called  "  smell."  But  only  one 
o(  these  innumerable  spermatozoa  is  chosen — namely,  the 
one  that  first  reaches  the  ovum  by  the  serpentine  motions  of 
its  tail,  and  touches  the  ovum  with  its  head.  At  the  spot 
.1 


\>Js4.fi 


Fig.  _o.  Impregnation  of  the  ovum  of  a  star-fish.  (From  Hertwig.) 
Only  ;i  small  part  of  the  surface  of  the  ovum  is  shown.  One  of  the  numerous 
spermatozoa  approaches  the  "impregnation  rise" (A),  touches  it  (BJ,  and 
then  penetrates  into  the  protoplasm  of  the  ovum  (  Cj. 

where  the  point  o(  its  head  touches  the  surface  of  the  ovum  the 
protoplasm  of  the  latter  is  raised  in  the  form  of  a  small  wart, 
the  "impregnation  rise"  (Fi,^r.  26  A).  The  spermatozoon 
then  bores  its  way  into  this  with  its  head,  the  tail  outside 
wriggling  about  all  the  time  (Fig.  26  />,  C).  Presently  the 
tail  also  disappears  within  the  ovum.  At  the  same  time  the 
ovum  secretes  a  thin  external  yelk-membrane  (Fig.  26  C), 
Starting  from  the  point  o\  impregnation;  and  this  prevents 
any  more  spermatozoa  from  entering. 

Inside  the  impregnated  ovum  we  now  see  a  rapid  series  o\ 
most  important  changes.  The  pear-shaped  head  o(  the 
sperm-cell,  or  the  "head  o\  the  spermatozoon,"  grows  larger 
and    rounder,   and    is    converted    into    the    male    pro-nucleus 


136  COXCEPTIOX 

(Fig.  27  $  k).  This  has  an  attractive  influence  on  the  fine 
granules  or  microsomata  which  are  distributed  in  the  proto- 
plasm of  the  ovum ;  they  arrange  themselves  in  lines  in  the 
figure  of  a  star  (cytulaster).  But  the  attraction  or  the 
"  affinity  "  between  the  two  nuclei  is  even  stronger.  They 
move  towards  each  other  inside  the  yelk  with  increasing 
speed,  the  male  (Fig.  28  j  k)  going  more  quickly  than  the 
female  nucleus  (e  k).  The  tiny  male  nucleus  takes  with  it 
the  radiating  mantle  which  spreads  like  a  star  about  it.  At 
last  the  two  sexual  nuclei  touch  (usually  in  the  centre  of  the 
globular  ovum),  lie  close  together,  are  flattened  at  the  points 
of  contact,   and  coalesce  into  a  common   mass.     The  small 


^<v^-?>-rv. 


Impregnation  of  the  ovum  of  the  sea-urchin.  ( From  Hertwig. )  In  Fig-. 

27  the  little  sperm-nucleus  (sh)  moves  towards  the  larger  nucleus  of  the  ovum 
(ck).  In  Fig.  28  they  nearly  touch,  and  arc  surrounded  by  the  radiating 
mantle  of  protoplasm. 

central  particle  of  nuclein  which  is  formed  from  this  combina- 
tion of  the  nuclei  is  the  stem-nucleus,  or  the  first  segmenta- 
tion nucleus  (archicaryon  or  eytulocaryon  J ;  the  new-formed 
cell,  the  product  of  the  impregnation,  is  our  stem-cell,  or 
"first  segmentation  sphere"  (cytttla  or  archicytos,  Fig.  29). 

Hence  the  one  essential  point  in  the  process  of  sexual 
reproduction  or  impregnation  is  the  formation  of  a  new  cell, 
the  stem-cell.  This  cytula  is  always  the  resultant  of  the  com- 
bination of  two  originally  different  cells,  the  female  ovum  and 
the  male  spermatozoon.  This  process  is  of  the  highest  impor- 
tance and  merits  our  closest  attention ;  all  that  happens  in  the 
later  development  of  this  first  cell  and  in  the  life  of  the  organism 
that  comes  of  it  is  determined  from  the  first  by  the  chemical 


(  <  >  VI  EPTIOX 


■37 


and  morphological  composition  of  the  stem-cell,  Its  nucleus 
and  its  body.  We  must,  therefore,  make  a  very  careful 
study  of  the  rise  and  structure  of  the  stem-cell. 

The  first  question  that  arises  is  as  to  the  behaviour  of  the 
two  different  active  elements,  the  nucleus  and  the  protoplasm, 
in  the  actual  coalescence.  It  is  obvious  that  the  nucleus 
plays  the  more  important  part  in  this.  Hence  Hertwig  puts 
his  theor)  of  conception  in  the  principle :  "Conception  consists 
in  the  copulation  of  two  cell-nuclei,  which  come  from  a  male 
and  a  female  cell."  And  as  the  phenomenon  of  heredity  is 
inseparably  connected  with  the  reproductive  process,  we  may 
further  conclude  that  these  two  copulating  nuclei  "convey  the 
characteristics  which  are  trans- 
mitted from  parents  to  offspring." 
In  this  sense  I  had  in  1866  (in  the 
ninth  chapter  of  the  Generelle 
Morphologie)  ascribed  to  the  re- 
productive nucleus  the  function 
ol  generation  and  heredity,  and 
to  the  nutritive  protoplasm  the 
duties  of  nutrition  and  adaptation. 
As,  moreover,  there  is  a  complete 
coalescence  of  the  mutually  attrac- 
ted nuclear  substances  in  concep- 
tion, and  the  new  nucleus  formed 
(the  stem-nucleus)  is  the  real 
starting-point  for  the  development  of  the  fresh  organism,  the 
further  conclusion  may  be  drawn  that  the  male  nucleus 
conveys  to  the  child  the  qualities  of  the  father,  and  the 
female  nucleus  the  features  oi  the  mother.  We  must  not 
forget,  however,  that  the  protoplasmic  bodies  of  the 
copulating  cells  also  fuse  together  in  the  act  of  impreg- 
nation ;  the  cell-body  of  the  invading  spermatozoon  (the 
trunk  and  tail  oi  the  male  ciliated  cell)  is  dissolved  in 
the  yelk  oi  the  female  ovum.  This  coalescence  is  not 
so  important  as  that  oi  the  nuclei,  but  it  must  not  be 
overlooked  ;  and,  though  this  process  is  not  so  well  known 
to    us,  we   see  clearly  at   least  the    formation  of  the    star-like 


Fig.  -a  Stem-cell  of  ey- 
tula  of  a  sea-urchin  (first- 
segmentation-cell,  or  impreg- 
nated ovum).  (From  Herhuig.) 
In  the  centre  of  the  globular  cell 
is  the  small  globular  stem-nucleus 
or  segmentation-nucleus  (fk). 


CONCEPTION 


figure  (the  radial  arrangement  of  the  microsomata  in  the 
plasma)  in  it  (Figs.  27-29). 

Mention  must  also  be  made  of  the  reciprocal  action  o\~  the 
cell-constituents  on  both  sides.  The  formation  of  the  proto- 
plasmic star  around  the  invading  male  nucleus,  and  after- 
wards round  the  copulated  stem-nucleus,  suggests  the  idea 
that  this  alone  has  an  active  influence  on  the  arrangement  of 
the  granules  and  threads  in  the  protoplasm.  However,  the 
reproductive  nucleus  itself  changes  its  size,  shape,  and  con- 
sistency, and  is  on  its  side  influenced,  from  the  conditions 
under  which  it  is  nourished,  by  the  nutritive  protoplasm. 
How  close  the  interaction  of  the  two  elements  is  can  be 
seen  at  once  from  the  above-mentioned  preliminary  processes 
of  the  maturing  of  the  ovum  before  impregnation,  and  from 
the  segmentation  processes  that  follow  it.  In  both  cases  we 
observe  the  complete  phenomena  of  caryokinesis  and  mitosis, 
which  are  found  always  in  indirect  cleavage,  and  which  reveal 
to  us  the  significant  interaction  of  cell-nucleus  and  cell-body. 
These  phenomena  have  also  been  called  caryolysis,  or  the 
"  dissolving  of  the  nucleus  in  the  protoplasm."  This  may  be 
granted  up  to  a  certain  point,  and  used  in  support  of  our 
monera  theory — for  the  belief  that  the  oldest  and  simplest 
organisms  were  innucleated  plastids,  and  that  the  real  unicel- 
lular forms  of  life  were  subsequently  developed  from  these  by 
the  cleavage  of  nucleus  and  cell-body.  (Cf.  the  nineteenth 
Chapter.) 

The  older  theories  of  impregnation  generally  went  astray 
in  regarding  the  large  ovum  as  the  sole  base  of  the  new 
organism,  and  only  ascribed  to  the  spermatozoon  the  role  of 
stimulating  and  originating  its  development.  The  stimulus 
which  it  gave  to  the  ovum  was  sometimes  thought  to  be 
purely  chemical  (a  catalytic  process),  at  other  times  rather 
physical  (on  the  principle  of  transferred  movement),  or 
again  quite  dualistic  (that  is,  a  mystic  and  transcendental 
process).  This  error  was  partly  due  to  the  imperfect  know- 
ledge at  that  time  of  the  facts  of  impregnation,  and  partly  to 
the  striking  difference  in  the  sizes  of  the  two  sexual  cells. 
Most  of  the  earlier  observers  thought  that  the  spermatozoon 


CONCEPTION 


did  not  penetrate  into  the  ovum.  And  even  when  this  had 
been  demonstrated,  the  spermatozoon  was  believed  to  dis- 
appear in  the  ovum  without  leaving  a  trace.  However,  the 
splendid  research  made  in  the  last  three  decades  with  the  finer 
technical  methods  of  our  time  has  completely  exposed  the 
error  o(  this.  It  lias  been  shown  that  the  tiny  sperm-cell  is 
not  subordinated  to,  but  co-ordinated  with,  the  large  ovum. 
The  nuclei  of  the  two  cells,  as  the  vehicles  o\  the  hereditary 
features  of  the  parents,  are  of  equal  physiological  importance. 

In  some  cases  we  have  succeeded  in  proving  that  the  mass 
of  the  active  nuclear  substance  which  combines  in  the  copula- 
tion o(  the  two  sexual  nuclei  is  orginally  the  same  for  both. 
Edward  Van  Beneden  has  shown  that  in  the  ovum  of  the 
horse  maw-worm  f  ascaria  megalocephcUa)  the  union  of  the 
two  sexual  nuclei  is  delayed  until  the  stem-cell  created  begins 
to  divide.  The  characteristic  nuclear  spindle  which  is  then 
formed,  and  which  falls  into  the  nuclei  of  the  two  first 
segmentation  daughter-cells,  is  formed  half  of  the  nucleus 
of  the  ovum  and  half  of  the  sperm-nucleus  ;  of  the  four 
"  daughter-loops "  of  the  segmentation  spindle  two  are  of 
male  and  two  of  female  origin. 

These  morphological  facts  are  in  perfect  harmony  with  the 
familiar  physiological  truth  that  the  child  inherits  from  both 
parents,  and  that  on  the  average  they  are  equally  distributed. 
I  say  "  on  the  average,"  because  it  is  well  known  that  a  child 
may  have  a  greater  likeness  to  the  father  or  to  the  mother  ; 
that  goes  without  saying,  as  far  as  the  primary  sexual 
characters  (the  sexual  glands)  are  concerned.  But  it  is  also 
possible  that  the  determination  of  the  latter — the  weighty 
determination  whether  the  child  is  to  be  a  boy  or  a  girl — 
depends  on  a  slight  qualitative  or  quantitative  difference  in 
the  nuclein  or  the  chromatic  nuclear  matter  which  comes 
from  both  parents  in  the  act  of  conception. 

The  striking  differences  of  the  respective  sexual  cells  in 
si/.e  and  shape,  which  occasioned  the  erroneous  views  *>t 
earlier  scientists,  are  easily  explained  on  the  principle  ol 
division  of  labour,  or  ergonomy.  The  inert,  motionless 
ovum  grows  in  size  according  to  the  quantity  o\~  provision   it 


COXCEPTIOX 


stores  up  in  the  form  of  nutritive  yelk  for  the  development  of 
the  germ.  The  active  swimming  sperm-cell  is  reduced  in 
size  in  proportion  to  its  need  to  seek  the  ovum  and  bore  its 
way  into  its  yelk.  These  differences  are  very  conspicuous  in 
the  higher  animals,  but  they  are  much  less  in  the  lower 
animals.  In  those  protists  (unicellular  plants  and  animals) 
which  have  the  first  rudiments  of  sexual  reproduction  the  two 
copulating  cells  are  at  first  quite  equal.  In  these  cases  the 
act  of  impregnation  is  nothing  more  than  a  sudden  growth, 
in  which  the  originally  simple  cell  doubles  its  volume,  and  is 
thus  prepared  for  reproduction  (cell-division).  Afterwards 
slight  differences  are  seen  in  the  size  of  the  copulating  cells  ; 
though  the  smaller  microspores  (or  microgonidia)  still  have 
the  same  shape  as  the  larger  macrospores  (or  macrogonidia). 
It  is  only  when  the  difference  in  size  is  very  pronounced  tli£ 
a  notable  difference  in  shape  is  found  :  the  sprightly  sperr 
cell  changes  more  in  shape  and  the  ovum  in  size. 

Quite  in  harmony  with  this  new  conception  of  /the 
equivalence  of  the  two  gonidia,  or  the  equal  physiological 
importance  of  the  male  and  female  sex-cells  and  their  equal 
share  in  the  process  of  heredity,  is  the  important  fact 
established  by  Hertwig  (1875),  that  in  normal  impregnation 
only  one  single  spermatozoon  copulates  with  one  ovum  ;  the 
membrane  which  is  raised  on  the  surface  of  the  yelk  imme- 
diately after  one  sperm-cell  has  penetrated  (Fig.  26  C) 
prevents  any  others  from  entering.  All  the  rivals  of  the 
fortunate  penetrator  are  excluded,  and  die  without.  But  if 
the  ovum  passes  into  a  morbid  state,  if  it  is  made  stiff  by  a 
lowering  of  its  temperature  or  stupefied  with  narcotics 
(chloroform,  morphia,  nicotine,  etc.),  two  or  more  sperma- 
tozoa may  penetrate  into  its  yelk-bcdy.  We  then  witness 
polyspermism.  The  more  Hertwig  chloroformed  the  ovum, 
ie  more  spermatozoa  were  able  to  bore  their  way  into  its 
unconscious  body. 

These  remarkable  facts  of  impregnation  are  also  of  the 
greatest  interest  in  psychology,  especially  as  regards  the 
theory  of  the  cell-soul,  which  I  consider  to  be  its  chief 
foundation.      All  the  phenomena  we  have  described  can  only 


CONCEPTION 


be  understood  and  explained  by  ascribing  a  certain  lower 
degree  of  psychic  activity  to  the  sexual  principles.  They  feel 
each  other's  proximity,  and  are  drawn  together  by  a  sensitive 
impulse  (probablj  related  to  smell)  ;  they  move  towards  each 
other,  and  do  not  rest  until  they  fuse  together.  Physiologists 
may  say  that  it  is  only  a  question  of  a  peculiar  physico- 
chemical  phenomenon,  and  not  a  psychic  action  ;  but  the  two 
cannot  be  separated.  Even  the  psychic  functions,  in  the 
strict  sense  of  the  word,  are  only  complex  physical  processes, 
or  "psycho-physical"  phenomena,  which  are  determined  in 
all  cases  exclusively  by  the  chemical  composition  of  their 
material  substratum. 

The  monistic  view  of  the  matter  becomes  clear  enough 
when  we  remember  the  radical  importance  of  impregnation  as 
regards  heredity.  It  is  well  known  that  not  only  the  most 
delicate  bodily  structures,  but  also  the  subtlest  traits  of  mind, 
are  transmitted  from  the  parents  to  the  children.  In  this  the 
chromatic  matter  of  the  male  nucleus  is  just  as  important  a 
vehicle  as  the  large  caryoplasmic  substance  of  the  female 
nucleus  ;  the  one  transmits  the  mental  features  of  the  father, 
and  the  other  those  of  the  mother.  The  blending  of  the  two 
parental  nuclei  determines  the  individual  psychic  character  of 
the  child. 

But  there  is  another  important  psychological  question — 
the  most  important  of  all — that  has  been  definitely  answered 
by  the  recent  discoveries  in  connection  with  conception. 
This  is  the  question  of  personal  immortality.  This  dogma, 
which  we  meet  in  the  most  varied  forms  among  uncivilised 
peoples,  occupies  an  important  place  also  in  the  higher 
conceptions  of  civilised  nations.  But  the  fact  that  it  is 
untenable  has  been  growing  clearer  and  clearer  during  the 
last  fifty  years,  chiefly  through  the  vast  progress  we  have 
made  in  comparative  morphology,  experimental  physiology, 
empirical  psychology,  psychiatry,  monistic  anthropology, 
and  ethnography.  However,  no  fact  throws  more  light  on  it 
and  refutes  it  more  convincingly  than  the  elementary  process 
of  conception  that  we  have  described.  For  this  copulation  ot 
the    two    sexual    nuclei    (Figs.    27-29)    indicates    the     precise 


CO.XCEPTIOX 


moment  at  which  the  individual  begins  to  exist.  All  the 
bodily  and  mental  features  of  the  new-born  child  are  the 
sum-total  of  the  hereditary  qualities  which  it  has  received  in 
reproduction  from  parents  and  ancestors.  All  that  man 
acquires  afterwards  in  life  by  the  exercise  of  his  organs,  the 
influence  of  his  environment,  and  education — in  a  word,  by 
adaptation — cannot  obliterate  that  general  outline  of  his 
being  which  he  inherited  from  his  parents.  But  this  heredi- 
tary disposition,  the  essence  of  every  human  soul,  is  not 
"eternal,"  but  "temporal";  it  comes  into  being  only  at  the 
moment  when  the  sperm-nucleus  of  the  father  and  the  nucleus 
of  the  maternal  ovum  meet  and  fuse  together. 

It  is  clearly  irrational  to  assume  an  "  eternal  life  without 
md  "  for  an  individual  phenomenon,  the  commencement  of' 
which  we  can  indicate  to  a  moment  by  direct  visual  observa- 
tion. But  the  unbroken  chain  of  plasma-movements  which 
we  comprise  under  the  title  of  a  man's  "  soul  "  is  just  such 
an  individual  phenomenon.  This  chain  of  molecular  move- 
ments begins  at  the  moment  when  the  paternal  nucleus  fuses 
with  the  maternal.  From  the  stem-nucleus  thus  produced  it 
is  transmitted,  in  the  repeated  segmentation,  to  all  the  similar 
cells  of  the  germinal  layer.  When  these  blastodermic  cells 
grow  into  the  two  primary  germinal  layers  of  the  gastrula, 
the  first  division  of  labour  in  the  cells  takes  place  ;  and  this 
continues  when  the  various  tissues  arise  from  them.  Later, 
in  man  and  the  higher  animals,  it  is  only  the  central  nerve- 
cells  which  are  the  primary  organs  of  psychic  life.  At  their 
death  the  mental  life  is  extinguished,  just  as  the  faculty  of 
vision  perishes  with  the  eye. 

We  often  hear  it  said  that  the  belief  in  immortality  is  an 
indispensable  foundation  of  religion  and  morality,  like  the 
belief  in  a  personal  God.  This  opinion  is  totally  opposed  to 
the  facts  of  history.  In  any  case  it  is  clear  that  all  that  is 
"personal  "  must  be  transitory,  a  mere  passing  phenomenal 
form  in  the  course  of  the  evolutionary  process.  Hence  it  is  a 
curious  error  to  speak,  as  Weismann  does,  of  the  immortality 
of  the  unicellular  beings.  The  unicellular  protists  are 
transitory    individuals    just    as     truly    as    the     multicellular 


CONCEPTION 


organisms,  to  which  man  belongs.     Ii  is  true  that  our  human 

soul  is  often  regarded  as  something  unique,  and  credited  with 
peculiar  powers  that  are  not  found  in  the  other  vertebrates. 
But  an  impartial  study  of  comparative  psychology  completely 
disposes  o\  this  illusion.  We  shall  see  that  the  special 
Organs  o(  man's  mental  life  are  evolved  in  just  the  same  way 
as  those  of  other  vertebrates. 

The  great  importance  of  the  process  of  impregnation  in 
answering  these  and  other  cardinal  questions  is  quite  clear. 
It  is  true  that  conception  has  never  been  studied  micro- 
scopically in  all  its  details  in  the  human  case — notwith- 
standing    its     occurrence 

A. 


at  every  moment  —  for 
reasons  that  are  obvious 
enough.  However,  the 
two  cells  which  need  con- 
sideration, the  female 
ovum  and  the  male  sper- 
matozoon, proceed  in  the 
case  of  man  in  just  the 
same  way  as  in  all  the 
other  mammals  ;  the 
human  foetus  or  embryo 
which  results  from  copula- 
tion has  the  same  form  as 
with  the  other  animals. 
Hence,  no  scientist  who 
is  acquainted  with  the  facts  doubts  that  the  processes  of 
impregnation  are  just  the  same  in  man  as  in  the  other 
animals. 

The  stem-cell  which  is  produced,  and  with  which  every 
man  begins  his  career,  cannot  be  distinguished  in  appearance 
from  those  of  other  mammals,  such  as  the  hare  (Fig.  p,o).  In 
the  case  of  man,  also,  this  stem-cell  differs  materially  from 
the  original  ovum,  both  in  regard  to  form  (morphologically), 
in  regard  to  material  composition  (chemically),  and  in  regard 
to  vital  properties  (physiologically).  It  comes  partly  from 
the    father   and    partly    from    the    mother.      Hence    it    is    nol 


Fig.  30.— Stem-cell  of  a  hare,  mag- 
nified joo  times.  In  the  centre  of  the 
granular  protoplasm  of  the  fertilised  ovum 
( </ j  is  seen  the  little,  bright  stem-nucleus. 
.  is  the  ovolemma,  with  a  mucous  mem- 
brane ( h).    s  are  dead  spermatozoa. 


CONCEPTION 


surprising  that   the  child  who   is  developed  from  it    inherits 
from  both  parents.1 

The  vital  movements  of  each  of  these  cells  form  a  sum  of 
mechanical  processes  which  in  the  last  analysis  are  due  to 
movements  of  the  smallest  vital  parts,  or  the  molecules  of  the 
living  substance.  If  we  agree  to  call  this  active  substance 
plasson  and  its  molecules  plastidules,  we  may  say  that  the 
individual  physiological  character  of  each  of  these  cells  is  due 
to  its  molecular  plastidule-movement.  Hence,  the plastidule- 
movement  of  the  cytula  is  the  resultant  of  the  combined  plasti- 
dule-nwvenients  of  the  female  ovum  and  the  male  sperm-cell. 
If  we  take  the  latter  two  to  be  the  side-lines  in  a  parallelogram 
of  forces,  the  plastidule-movement  of  the  stem-cell  is  its 
diagonal.  I  have  shown,  in  my  essay  on  "The  Perigenesisof 
the  Plastidule,  or  the  Wave-movement  of  the  Vital  Particles  " 
(1876),  the  importance  of  this  view  for  a  mechanical  explana- 
tion of  the  elementary  processes  of  evolution. 


1  The  plasson  of  the  stem-cell  or  cytula  may,  from  the  anatomical  point  of 
view,  be  regarded  as  homogeneous  and  structureless,  like  that  of  the  monera. 
This  is  not  inconsistent  with  our  hypothetical  ascription  to  the  plastidules  (or 
molecules  of  the  plasson)  of  a  complex  molecular  structure.  The  complexity 
of  this  is  the  greater  in  proportion  to  the  complexity  of  the  organism  that  is 
developed  from  it  and  the  length  of  the  chain  of  its  ancestry,  or  to  the  multi- 
tude of  antecedent  processes  of  heredity  and  adaptation. 


FIRST  TABLE 

SUMMARY    OF    THE    COMPOSITION    OF    THE 

ORGANIC   CELL 

(The  Elementary  Organism) 


Constituents  of 
the  FirsI  Order. 


Constituents  of 
the  Second  Order. 


Constituents  oi 
the  Third  Order. 


Constituents  of 
the  Fourth  Order. 


I.  Cell-nucleus, 
or  Caryon. 

Originally 
composed  of 
homogeneous 
nuclear  matter 
(caryoplasm). 


..   Caryobasis. 

Chief  Nuclear 

Mass 

(stiff  definite 
nuclear  matter  I. 

2.   Caryolymph. 

Nuclear  Sap 

(soft  formless 

matter). 


I.    Caryomitntna. 

Nuclear 

Skeleton. 

made  up  o( 

A.   Chromatin 

(coloured  nuclear 

matter); 

B.  Achromatin 
(colourless  nuclear 

matter) ; 

C.  Centrosoma 
(colourless  central 

corpuscles). 


'a)  Nucleolinus, 
nuclear  point. 

/>)  Nucleolus, 
nuclear  cor- 
puscles. 

el  Caryomita, 
nuclearth  reads. 

d)  Caryotheka, 
nuclear  mem- 
brane. 


/     i.   Cytomitoma, 

i.   Filar  matter,  or 

i.  Protoplasma. 

Cell-skeleton, 

spongroplasm , 

Active  (living) 

made  up  of  cyto- 

Mesh-work  or 

cell-matter. 

mita  or  proto- 
{   plasmic  threads. 

honeycomb 

2.   Metaplasma. 

a)  Paraplasma, 

II.  Cell-body 

Passive  (dead) 

Definite  inter- 

[cetteus  or  cyto- 

cell-matter 

filar  matter. 

plasm). 

(plasma-products) 

2.  A.   Internal 

h)  Aficrosomata,or 

Originally 

In   very  young 

plasma-products 

granula, 
granules  of 

composed  of 

cells  of  primary 

(stored  within 

plasma. 

homogeneous 

composition  there 

the  protoplasm |. 

c)  Lipsomata, 

cellular  matter 

is  nometaplasm ; 

granules  of  tat. 

( cytoplasm ). 

the    whole    cell- 

d)  Cytolymph, 

body  consist  s 

cell-sap. 

solely  of  homo- 

a) Cytotheta, 

geneous    proto- 

2.   B.    External 

membrane  ol 

plasm. 

plasma  products 
(extruded  from 

cell. 

b)  Intercellular 

1 

the  protoplasm). 

matter. 

CHAPTER  VIII. 

THE  GASTR^EA  THEORY1 

First  changes  after  the  impregnation  of  the  ovum.  The  original  or  palingenetic 
form  of  segmentation.  Nature  of  the  segmentation-process.  Repeated 
cleavage  of  the  stem-cell.  Formation  of  several  segmentation  spheres  or 
blastomeres.  Mulberry-like  structure,  or  morula.  Blastula.  Germinal 
membrane  or  blastoderm.  Folding  of  the  blastula.  Formation  of  the 
gastrula.  Depula,  transition  from  the  blastula  to  the  gastrula.  Primitive 
gut  and  primitive  mouth.  The  two  primary  germinal  layers  :  ectoderm 
(epiblast)  and  entoderm  (hypoblast).  Differences  between  their  cells. 
Similarity  of  the  original  gastrulation  in  the  most  distant  groups  of  the 
animal  world.  The  gastrulation  of  the  amphioxus  ;  transition  from  the 
primary  (uni-axial)  to  the  secondary  (bi-lateral  or  tri-axial)  form  of  the 
gastrula.  Bending  of  the  chief  axis.  Flattening  of  the  hinder  side,  large 
growth  of  the  fore-side.  The  secondary,  modified,  or  cenogenctic  forms 
of  gastrulation.  Significance  and  unequal  distribution  of  the  yelk.  Total 
and  partial  cleavage.  Holoblastie  and  meroblastie  ova.  Disc-like  cleavage 
and  disc-gastrula  :  fishes,  reptiles,  birds.  Superficial  cleavage  and  globular 
gastrula :  articulata.  Permanent  two-layered  structure  of  the  lower 
animals.  The  two-layered  primitive  stem-form  :  gastrffia.  Homology  of 
the  two  primary  germinal  layers. 

There  is  a  substantial  agreement  throughout  the  animal 
world  in  the  first  changes  which  follow  the  impregnation  of 
the  ovum  and  the  formation  of  the  stem-cell ;  they  begin  in  all 
cases  with  the  segmentation  of  the  ovum  and  the  formation  of 
the  germinal  layers.  The  only  exception  is  found  in  the 
protozoa,  the  very  lowest  and  simplest  forms  of  animal  life  ; 
these  remain  unicellular  throughout  life.  To  this  group 
belong  the  amoebae,  gregarina^,  rhizopods,  infusoria,  etc. 
As  their  whole  organism  consists  of  a  single  cell,  they  can 
never  form  germinal  layers,  or  definite  strata  of  cells.  But 
all    the    other   animals — all    the    tissue-forming    animals,    or 

1  Cf.  E.  Ray-Lankester's  essays  "  On  the  Primitive  Cell-layers  01  the 
Embryo  as  the  Basis  of  Genealogical  Classification  of  the  Animals  "  (Ann. 
Mag.  Nat.  Hist.,  vol.  xi.,  1873)  and  '•  Notes  on  the  Embryology  and  Classifi- 
cation of  the  Animal  Kingdom  "  ( Quarterly  Journal  of  Microscopic  Science,  vol. 
xvii. ,  1877),  and  Francis  Balfour's  Manual  of  Comparative  Embryology,  and 
"  On  the  Structure  and  Homology  of  the  Germinal  Layers  of  the  Embryo  " 
( Quart.  Journal  of  Micros.  Science.  1SS0). 
146 


THE  GAST8AVA   THEORY 


metazoa,  as  we  call  them,  in  contradistinction  to  the  protozoa 
— construct  real  germinal  layers  by  the  repeated  cleavage  of 
the  impregnated  ovum.  This  we  find  in  the  lower  cnidaria 
and  worms,  as  well  as  in  the  more  highly-developed  molluscs, 
echinoderms,  articulates,  and  vertebrates. 

In  all  these  metazoa,  or  multicellular  animals,  the  chief 
embryonic  processes  are  substantially  alike,  although  they 
often  seem  to  a  superficial  observer  to  differ  considerably. 
The  stem-cell  that  proceeds  from  the  impregnated  ovum 
always  passes  by  repeated  fission  into  a  number  of  simple 
cells.  These  cells  are  all  direct  descendants  of  the  stem-cell, 
and  are,  for  reasons  we  shall  see  presently,  called  segmenta- 
tion-cells, or  segmentation-spheres  (blaslomera  or  segmen- 
tella).  The  repeated  cleavage  of  the  stem-cell,  which  gives 
rise  to  these  segmentation-spheres,  has  long  been  known  as 
••  segmentation."  Sooner  or  later  the  segmentation-cells  join 
together  to  form  a  round  (at  first,  globular)  embryonal  sphere 
(bias  tula  J;  they  then  form  into  two  very  different  groups, 
and  arrange  themselves  in  two  separate  strata — the  two 
primary  germinal  layers.  These  enclose  a  digestive  cavity, 
the  primitive  gut,  with  an  opening,  the  primitive  mouth. 
We  give  the  name  of  the  gastrula  to  the  important  embryonic 
form  that  has  these  primitive  organs,  and  the  name  of  gastru- 
lation  to  the  formation  o(  it.  This  ontogenetic  process  has  a 
very  great  significance,  and  is  the  real  starting-point  of  the 
construction  of  the  multicellular  animal  body. 

The  fundamental  embryonic  processes  of  the  cleavage  of 
the  ovum  and  the  formation  of  the  germinal  lavers  have  been 
very  thoroughly  studied  in  the  last  thirty  years,  and  their 
real  significance  has  been  appreciated.  They  present  a 
striking  variety  in  the  different  groups,  and  it  was  no  light 
task  to  prove  their  essential  identity  in  the  whole  animal 
world.  But  since  I  formulated  the  gastraja  theory  in  1S72, 
and  afterwards  (1875)  reduced  all  the  various  forms  of 
segmentation  and  gastrulation  to  one  fundamental  type,  their 
identity  may  be  said  to  have  been  established.  We  have 
thus  mastered  the  law  of  unity  which  governs  the  first 
embryonic  processes  in  all  the  animals. 


THE  GASTR.EA    THEORY 


Man  is  like  all  the  other  higher  animals,  especially  the 
apes,  in  regard  to  these  earliest  and  most  important  pro- 
cesses. As  the  human  embryo  does  not  essentially  differ, 
even  at  a  much  later  stage  of  development — when  we  already 
perceive  the  cerebral  lobes,  the  eyes,  ears,  gill-arches,  etc. 
— from  the  similar  forms  of  the  other  higher  mammals 
(cf.  Plate  XIII.,  first  row),  we  may  confidently  assume  that 
they  agree  in  the  earliest  embryonic  processes,  segmentation 
and  formation  of  germinal  layers.  This  has  not  yet,  it  is 
true,  been  established  by  observation.  We  have  never  yet 
had  occasion  to  dissect  a  woman  immediately  after  impregna- 
tion and  examine  the  stem-cell  or  the  segmentation-cells  in 
her  oviduct.  However,  as  the  earliest  human  embryos  (in 
the  form  of  embryonal  spheres)  we  have  examined,  and  the 
later  and  more  developed  forms,  agree  with  those  of  the  hare, 
dog,  and  other  higher  mammals,  no  reasonable  man  will 
doubt  but  that  the  segmentation  and  formation  of  layers  are 
the  same  in  both  cases,  as  Figs.  12-17  on  Plate  H-  represent. 

But  the  special  form  of  segmentation  and  layer-formation 
which  we  find  in  the  mammal  is  by  no  means  the  original, 
simple,  palingenetic  form.  It  has  been  much  modified  and 
cenogenetically  altered  by  a  very  complex  adaptation  to 
embryonic  conditions.  We  cannot,  therefore,  understand  it 
altogether  in  itself.  In  order  to  do  this,  we  have  to  make  a 
comparative  study  of  segmentation  and  layer-formation  in  the 
animal  world  ;  and  we  have  especially  to  seek  the  original, 
palingenetic  form  from  which  the  modified  cenogenetic  form 
has  gradually  been  developed. 

This  original  palingenetic  form  ot  segmentation  and 
layer-formation  is  found  to-day  in  only  one  case  in  the 
vertebrate-stem  to  which  man  belongs — the  lowest  and  oldest 
member  of  the  stem,  the  wonderful  lancelet  or  ampbioxus 
(cf.  Chapters  XVI.  and  XVII.,  and  Plates  XVIII.  and  XIX.). 
But  we  find  a  precisely  similar  palingenetic  form  of  embryonic 
development  in  the  case  of  many  of  the  invertebrate  animals, 
as,  for  instance,  the  remarkable  ascidia,  the  pond-snail 
(limnceusj,  the  arrow-worm  (sagit/a),  and  many  of  the 
echinoderms  and  cnidaria,  such  as  the  ordinary  star-fish  and 


muSSEST 


lIBMk, 


/'///•;  GASTRjEA  theory 


sea-urchin,  many  of  the  medusae  and  corals,  and  the  simpler 
sponges  '  olyiithus ,.  We  may  take  as  an  illustration  the 
palingenetic  segmentation  and  germinal  layer-formation  in  an 
eight-fold  insular  coral,  which  I  discovered  in  the  Red  Sea,  and 
described  in  my  Arabische  Korallen  as  monoxenta  Darwinti. 

The  impregnated  ovum  of  this  coral  (Fig.  31  A,  B)  first 
splits  into  two  equal  cells  (C).  First,  the  nucleus  of  the 
Stem-cell  and  the  dependent  centrosoma  divide  into  two 
halves.  These  recede  from  and  repel  each  other,  and  act 
as  centres  of  attraction  on  the  surrounding  protoplasm  ;  in 
consequence  of  this,  the  protoplasm  is  constricted  by  a  circular 
furrow,  and,  in  turn,  divides  into  two  halves.  Each  of  the 
two  segmentation-cells  thus  produced  splits  in  the  same  way 
into  two  equal  cells,  and,  in  fact,  the  plane  of  cleavage  of  the 
latter  two  lies  vertically  on  that  of  the  first  (Fig.  D).  The 
four  familiar  segmentation-cells  (grand-daughters  of  the 
stem-cell)  lie  in  one  plane.  Now,  however,  each  of  them 
sub-divides  into  two  equal  halves,  the  cleavage  of  the  nucleus 
again  preceding  that  of  the  surrounding  protoplasm.  The 
eight  cells  which  thus  arise  break  into  sixteen,  these  into 
thirty-two,  and  then  (each  being  constantly  halved)  into  sixty- 
lour,  128,  and  soon.1  The  final  result  of  this  repeated  cleavage 
is  the  formation  of  a  globular  cluster  of  similar  segmentation- 
cells,  which  we  call  the  mulberry-formation  or  morula.  The 
cells  are  thickly  pressed  together  like  the  parts  of  a  mulberry  or 
blackberry,  and  this  gives  a  lumpy  appearance  to  the  surface 
of  the  sphere  (Fig.  E).      [Cf.  also  Fig.  3  on  Plate  II.]2 

When  the  cleavage  is  thus  ended,  the  mulberry-like  mass 
changes  into  a  hollow  globular  sphere.  Watery  fluid  or 
jelly  gathers  inside  the  globule  ;  the  segmentation   cells  are 

'  Tin'  number  of  blastomeres  or  segmentation-cells  increases  geometrically 
in  the  original  gastrulation,  or  the  purest  palingenetic  form  of  cleavage. 
However,  in  different  archiblastic  animals  the  number  reaches  a  different 
height,  so  that  the  morula,  and  also  the  blastula,  may  consist  sometimes  of 
thirty-two,  sometimes  of  sixty-four,  and  sometimes  ol  128,  or  more,  rolls. 

•'  The  segmentation-cells  which  make  up  the  morula  after  tin-  close  oi  the 
palingenetic  cleavage  seem  usually  to  in-  quite  similar,  ami  to  present  no 
morphological  differences  as  to  size,  form,  and  composition.  That,  however, 
does  not  prevent  them  from  differentiating  into  animal  and  vegetative  rolls 
even  during  the  cleavage,  as  Figs.  2  and  ,',  on  Plate  II.  indicate. 


THE  GASTR.EA    THEORY 


Fig.  31.—  Gastrulation  Of  a  <ZQVa\(monoxenia  Darminii).  A,  B,  stem- 
cell  (cytula)  or  impregnated  ovum.  In  Fig-.  A  (immediately  after  impregnation} 
the  nucleus  is  invisible.  In  Fig.  B  (a  little  later)  it  is  quite  clear.  C  two 
segmentation-cells.       D     four     segmentation-cells.       E     mulberry-formation 

(morula).  F  embryonal  sphere  (blastula).  G  embryonal  sphere  (transverse 
section).  H  tufted  embryo  (depula,  or  hollowed  embryonal  sphere) — transverse 
section.  I  gastrula — longitudinal  section.  K  gastrula,  or  cup-sphere,  external 
appearance.  , 


THE  GASTRMA    THEORY  151 

loosened,  and  all  rise  to  the  surface.  There  they  are  flattened 
by  mutual  pressure,  and  assume  the  shape  of  truncated 
pyramids,  and  arrange  themselves  side  by  side  in  one  regular 
layer  (Figs.  F,  G).  This  layer  of  colls  is  called  the  germinal 
membrane  (blastoderm)  ;  the  homogeneous  cells  which  com- 
pose its  simple  structure  are  called  blastodermic  cells  f  cclluUv 
blastoderm iac  1  ;  and  the  whole  hollow  sphere,  the  walls  of 
which  are  made  of  the  preceding,  is  called  the  bias  tula,  or 
blastosphere  (or  vesicula  b/astoclerin tat).1 

In  the  case  of  our  coral,  and  of  many  other  lower  forms 
of  animal  life,  the  young  embryo  begins  at  once  to  move 
independently  and  swim  about  in  the  water.  A  fine,  long, 
thread-like  process,  a  sort  of  whip  or  lash,  grows  out  of  each 
blastodermic  cell,  and  this  independently  executes  vibratory 
movements,  slow  at  first,  but  quicker  after  a  time  (Fig.  F). 
In  this  way  each  blastodermic  cell  becomes  a  ciliated  cell. 
The  combined  force  of  all  these  vibrating  lashes  causes  the 
whole  blastula  to  move  about  in  a  rotatory  fashion.  In  many 
other  animals,  especially  those  in  which  the  embryo  developes 
within  enclosed  membranes,  the  vibratory  ciliated  cells  are 
only  formed  at  a  later  stage,  or  even  not  formed  at  all.  The 
blastosphere  may  grow  and  expand  by  the  blastodermic  cells 
(at  the  surface  of  the  sphere)  dividing  and  increasing,  and 
more  fluid  is  secreted  in  the  internal  cavity.  There  are  still 
to-day  some  organisms  that  remain  throughout  life  at  the 
structural  stage  of  the  blastula — hollow  vesicles  that  swim 
about  bv  a  ciliary  movement  in  the  water,  the  wall  of 
which  is  composed  of  a  single  layer  of  cells,  such  as  the 
volvox,  the  magosphsera,  synura,  etc.  We  shall  speak 
further  oi  the  great  phvlogenetie  significance  of  the  fact  in 
the  nineteenth  Chapter. 

A  very  important  and  remarkable  process  now  follows — 
namely,  the  curving  of  the  blastula  (invaginatio  blastula, 
Fig.  II).      The  vesicle  with  a  single  layer  of  cells  for  wall   is 

1  Tin/  blastula  of  the  lower  animals  must  not  be  confused  with  the  very 
different  blastula  of  the  mammal,  which  is  properly  called  the  gastrocystis  or 
blastocyst  is.  This  Mitogenetic  gastrocystis  and  the  palingenetic  blastula  are 
sometimes  very  wrongly  comprised  under  the  common  name  ol  blastula  or 
vesicula  blastodermica. 


THE  GASTR.EA    THEORY 


converted  into  a  cup  with  a  wall  of  two  layers  of  cells  (cf. 
Figs.  G,  H,  I).  A  certain  spot  at  the  surface  of  the  sphere 
is  flattened,  and  then  bent  inward.  This  depression  sinks 
deeper  and  deeper,  growing  at  the  cost  of  the  internal  cavity. 
The  latter  decreases  as  the  hollows  deepen.  At  last  the 
internal  cavity  disappears  altogether,  the  inner  side  of  the 
blastoderm  (that  which  lines  the  depression)  coming  to  lie 
close  on  the  outer  side.  At  the  same  time,  the  cells  of  the 
two  sections  assume  different  sizes  and  shapes  ;  the  inner 
cells  are  more  round  and  the  outer  more  oval  (Fig.  I).  In  this 
way  the  embryo  takes  the  form  of  a  cup  or  jar-shaped  body, 
with  a  wall  made  up  of  two  layers  of  cells,  the  inner  cavity 
of  which  opens  to  the  outside  at  one  end  (the  spot  where  the 
depression  was  originally  formed).  We  call  this  very  impor- 
tant and  interesting  embryonic  form  the  "cup-embryo"  or 
"cup-larva"  {gastrula,  Fig.  31,  I  longitudinal  section, 
K  external  view).1 

I  have  in  my  Natural  History  of  Creation  given  the  name 
of  "  tufted  embryo  "  or  depula  to  the  remarkable  intermediate 
form  which  appears  at  the  passage  of  the  blastula  into  the 
gastrula  :  "  In  this  intermediate  stage  there  are  two  cavities 
in  the  embryo — the  original  cavity  fblastocoelj  which  is  dis- 
appearing, and  the  primitive  gut-cavity  (progaster  )  which  is 
forming.  The  one  grows  at  the  expense  of  the  other  ;  though 
in  many  of  the  other  metazoa  a  relic  of  the  inner  cavity 
remains,  and  may  form  a  'false  body-cavity'  (pseiidocccl J.  This 
is  sometimes  rather  large,  and  is  often  called  the  '  primary 
body-cavity  '  of  the  metazoa,  in  opposition  to  the  '  secondary 
body-cavity,'  or  enteroccel,  which  developes  afterwards  out  of 
the  primitive  gut  in  the  vertebrates  "  (cf.  Chapter  X.). 

I  regard  the  gastrula  as  the  most  important  and  significant 
embryonic  form  in  the  animal  world.  In  all  real  animals 
(that  is,  excluding  the   unicellular  protists)  the  segmentation 

1  I  expounded  the  idea  of  the  gastrula  in  my  monograph  on  the  sponges  in 
1872.  I  already  laid  stress  on  "  the  extreme  importance  of  the  gastrula  in  the 
general  phytogeny  of  the  animal  kingdom  ":  "the  fact  that  this  larva-form  is 
found  in  the  most  different  animal  stems  has,  in  my  opinion,  a  significance  that 
it  is  impossible  to  exaggerate,  and  gives  a  clear  proof  of  the  common  origin 
oi'  all  from  the  gfastrsea." 


TlIK  GASTK.EA    THEORY 


oi  the  ovum  produces  either  a  pure,  primitive,  palingenetic 
gastrula  (Fig.  3 i  I,  K)  or  an  equally  instructive  cenogenetic 
form,  which  has  been  developed  in  time  from  the  first,  and 
can  immediately  he  reduced  to  it.      It  is  certainly  a  fact  of  the 


Fig,  32  f.//— Gastrula  of  a  very  simple  primitive-gut  animal, 
or  gastrsead  (gastrophysema).     (HaeckeL) 

FlG.  33  ( li ).—  Gastrula  Of  a  worm  (sagitta,  arrow-worm).  (From 
K&walevsty. ) 

Fig.  34  ( C >.  Gastrula  of  an  eehinoderm  (star-fish,  uraster),  not  com- 
pletely Folded  in  (depula).    (From  Alexander  Agassis.  1 

Fig.  35  ( D  ).—  Gastrula  of  an  arthropod  (primitive  crab,  nauplius) 
(as  34). 

Ftg.  36  (E).—  Gastrula  of  a  mollusc  (pond-snail,  limtueus).  (From 
Karl  RabL ) 

Fig.  si(F).  Gastrula  of  a  vertebrate  (lancelet,  amphioxus).  (From 
Kowalevsky.)    (Front  view.) 

In  each  figure  </  i-,  the  primitive-gut  cavity,  »/  primitive  mouth,  s  segmen- 
tation-cavity, 1  entoderm  (gut-layer),  e  ectoderm  (skin-layer). 


greatest  interest  and  instructiveness  that  animals  of  the  most 
different  stems — vertebrates  and  tunicates,  molluscs  and 
articulates,  echinoderms  and  annelids,  cnidaria  and  sponges — 


THE  GASTR.EA   THEORY 


proceed  from  one  and  the  same  embryonic  form.  In  illustra- 
tion I  give  a  few  pure  gastrula  forms  from  various  groups  of 
animals  (Figs.  32-37,  explanation  given  above). 

In  view  of  this  extraordinary  significance  of  the  gastrula, 
we  must  make  a  very  careful  study  of  its  original  structure. 
As  a  rule,  the  typical  gastrula  is  very  small,  being  invisible  to 
the  naked  eye,  or  at  the  most  only  visible  as  a  fine  point  under 
very  favourable  conditions,  and  measuring  generally  ^u  to  T\ 
of  a  millimetre  (less  frequently  \  to  1,  or  even  more)  in 
diameter.  In  shape  it  is  usually  like  a  roundish  drinking- 
cup.  Sometimes  it  is  rather  oval,  at  other  times  more 
ellipsoid  or  spindle-shaped  ;  in  some  cases  it  is  half  globular, 
or  even  almost  globular,  and  in  others  lengthened  out,  or 
almost  cylindrical.  The  geometrical  type-form — a  single 
axis  with  two  different  poles — is  very  characteristic.  This 
axis  is  the  long  axis  or  chief  axis  of  the  subsequent  uni-axial 
body;  one  pole  is  the  mouth-pole  (oral  pole),  and  the  other 
the  contra-mouthpole  (aboral  pole).  In  the  bilateral  animals, 
or  higher  animals  with  right  and  left  similar  halves  to  the 
structure,  the  cenogenetically  modified  gastrula  usually 
assumes  a  bilateral  (and  tri-axial)  form  at  an  early  stage 
(Fig.  41).  The  gastrula  is  distinguished  very  sharply  by 
this  uni-axial,  or  monaxial,  form  from  the  globular  blastula 
and  morula,  in  which  all  the  axes  of  the  body  are  alike.  The 
transverse  section  of  the  primary  gastrula  is  round. 

I  give  the  name  of  primitive  gut  1  progaster )  and 
primitive  mouth  (prostoma)  to  the  internal  cavity  of  the 
gastrula-body  and  its  opening ;  because  this  cavity  is  the 
first  rudiment  of  the  digestive  cavity  of  the  organism,  and 
the  opening  originally  served  to  take  food  into  it.  Naturally, 
the  primitive  gut  and  mouth  change  very  considerably  after- 
wards in  the  various  classes  of  animals.  In  most  of  the 
cnidaria  and  many  of  the  annelids  (worm-like  animals)  they 
remain  unchanged  throughout  life.  But  in  most  of  the 
higher  animals,  and  so  in  the  vertebrates,  only  the  larger 
central  part  of  the  later  alimentary  canal  developes  from  the 
primitive  gut  ;  the  later  mouth  is  a  fresh  development,  the 
primitive   mouth   disappearing  or   changing    into    the    anus. 


THE  GASTRJEA   THEORY 


We  must  therefore  distinguish  carefully  between  the  primi- 
tive gut  and  mouth  oi'  the  gastrula  and  the  later  alimentary 
canal  and  mouth  of  the  fully  developed  vertebrate.1 

The  two  layers  o\  cells  which  line  the  gut-cavity  and 
compose  its  wall  arc  o(  extreme  importance.  These  two 
layers,  which  are  the  sole  builders  of  the  whole  organism, 
arc  no  other  than  the  two  primary  germinal  layers,  or  the 
primitive  germ-layers  fblastophyllaj.  I  have  spoken  in  the 
introductory  section  (Chapter  III.)  of  their  radical  importance. 
The  outer  stratum  is  the  skin-layer,  or  ectoderm  (Figs. 
32-37*);  the  inner  stratum  is  the  gut-layer,  or  entoderm  (/). 
The  former  is  often  also  called  the  ectoblast,  or  epiblast,  and 
the  latter  the  endoblast,  or  hypoblast.  From  these  two 
primary  germinal  layers  alone  is  developed  the  entire  organism 
of  all  the  metazoa  or  multicellular  animals.  The  skin-layer 
forms  the  external  skin,  the  gut-layer  forms  the  internal  skin  or 
lining  of  the  body.  Between  these  two  germinal  layers  are 
afterwards  developed  the  middle  germinal  layer  (mesoderma) 
and  the  body-cavity  fcoelosomaj  filled  with  blood  or  lymph. 

The  two  primary  germinal  layers  were  first  distinguished 
by  Pander  in  1S1 7  in  the  incubated  chick,  the  outer  being 
called  the  serous,  and  the  inner  the  mucous,  layer  (p.  ^9). 
But  their  full  significance  was  first  realised  by  Baer,  who 
called  the  first  the  animal,  and  the  second  the  vegetative, 
layer  in  his  classical  work  on  embryology  (1828).  These 
names  are  suitable  enough  in  the  sense  that  the  animal  organs 
of  sensation — the  skin,  nerves,  and  sense-organs — are  formed 
chieflv  (if  not  exclusively)  from  the  outer  layer;  and  the 
vegetal  organs  of  nutrition  and  reproduction,  especially  the 
alimentary  canal  and  the  blood-vessels,  are  formed  chiefly 
from  the   inner    layer.      Twenty  years   later   (1849)   Huxley 

■  My  distinction  (1872)  between  the  primitive  gu(  and  mouth  and  the  later 
permanent  stomach  (metagaster)  and  mouth  (metastoma)  has  been  much 
criticised;  bul  ii  is  as  much  justified  as  the  distinction  between  the  primitive 
kidneys  and  the  permanent  kidneys.  Professor  K.  Ray-Lankester  suggested 
three  years  afterwards  (1875)  the  name  archenteron  for  the  primitive  gut,  and 

blastopOTUS  for  the  primitive  mouth.  An  interesting  theory  of  the  mouth  has 
lately  been  put  forward  by  Daniele  Rosa  {of  Moilena)  in  his  essay,  "II 
canale  neurenterico  ed  il  blastopore  anale"  (BoUetino  Z00U  di  Torino.  \o. 
I46,  1903). 


'56 


THE  GASTR.EA    THEORY 


pointed  out  that  in  many  of  the  lower  zoophyta,  especially 
the  medusae,  the  whole  body  consists  throughout  life  of  these 
two  primary  germinal  layers.  Soon  afterwards  (1853) 
Allman  introduced  the  names  which  have  come  into  genera 
use  ;  he  called  the  outer  layer  the  ectoderm  ("  outer-skin  "), 
and  the  inner  the  entoderm  ("  inner-skin  ").  But  in  1867  it 
was  shown,  particularly  by  Kowalevsky,  from  comparative 
observation,  that  even  in  invertebrates,  also,  of  the  most 
different  classes  —  annelids,  molluscs,  echinoderms,  and 
articulates — the   body   is   developed    out    of    the   same    two 


Fig.  3S.— Gastrula  of  a  lower  sponge  (olynthus).    A  external  view,  B 

longitudinal  section  through  the  axis, g  primitive  gut-cavity,  0  primitive  mouth- 
aperture,  i  inner  cell-layer  (entoderm,  endoblast,  gut-layer),  e  external  cell- 
layer  (outer  germinal  layer,  ectoderm,  eetoblast,  or  skin-layer). 


primary  layers.  Finally,  I  discovered  them  (1872)  in  the 
lowest  tissue-forming  animals,  the  sponges,  and  proved  in 
my  gastrasa  theory  that  these  marginal  layers  must  be 
regarded  as  identical  or  homologous  throughout  the  animal 
world,  from  the  sponges  and  corals  to  the  insects  and  verte- 
brates, including  man.  This  fundamental  "  homology  of  the 
primary  germinal  layers  and  the  primitive  gut "  has  been 
confirmed  during  the  last  thirty  years  by  the  careful  research 
of  many  able  observers,  and  is  now  pretty  generally  admitted 
for  the  whole  of  the  metazoa. 

As   a    rule,    the   cells   which    compose    the    two   primary 


THE  GASTRJEA   THEORY 


germinal  layers  show  appreciable  differences  even  in  the 
gastrula  stage.  Generally  (if  not  always)  the  cells  of  the 
skin-layer  or  ectoderm  (Figs.  38c,  390)  are  the  smaller,  more 
numerous,  and  clearer;  while  the  cells  of  the  gut-layer,  or 
entoderm  (/),  are  larger,  less  numerous,  and  darker.  The 
protoplasm  of  the  ectoderm  cells  is  clearer  and  firmer  than 
the  thicker  and  softer  cell-matter  of  the  entoderm-cells  ;  the 
latter  are,  as  a  rule,  much  richer  in  yelk-granules  (albumen 
and  fatty  particles)  than  the  former.  Also  the  cells  of  the 
gut-layer  have,  as  a  rule,  a  stronger  affinity  for  colouring 
matter,  and  take  on  a  tinge  in  a  solution  of  carmine,  aniline, 
etc.,  more  quickly  and  appreciably  than  the  cells  of  the  skin- 
laver.  The  nuclei  of  the  entoderm-cells  are  usually  roundish, 
while  those  of  the  ectoderm-cells  are  oval. 

These  physical,  chemical,  and  morphological  differences 
in  the  two  germinal  layers,  corresponding  to  their  physio- 
logical contrast,  are  of  interest  as  showing  us  the  first 
and  oldest  process  of  differentiation  in  the  animal  body. 
The  skin-laver  (blastoderm),  which  forms  the  wall  of  the 
globular  blastula  (Fig.  31  F,  G),  consists  o\  a  single  stratum 
of  homogeneous  cells.  These  blastodermic  cells  are  at  first 
very  regular  and  of  similar  construction,  and  exactly  alike  in 
si/e,  shape,  and  texture.  They  are  usually  flattened  by 
mutual  pressure,  and  very  often  strictly  hexagonal.  They 
make  the  first  tissue  of  the  meta/.oon-organism,  a  simple  cell- 
pavement  or  epithelium.  The  homogeneity  of  these  cells 
disappears  sooner  or  later  during  the  curving  of  the  blasto- 
sphere.  The  cells  which  form  its  inner  concave  part  (the 
subsequent  entoderm)  assume,  as  a  rule,  during  the  very 
process  of  folding  (Fig.  31  1 1 ),  different  features  from  those 
which  constitute  the  outer  convex  part  (the  subsequent  ecto- 
derm). When  the  folding-process  is  complete,  very  striking 
histological  differences  between  the  cells  of  the  two  layers  are 
found  (Fig.  39).  The  tiny,  light  ectoderm-cells  (e)  are 
sharplv  distinguished  from  the  larger  and  darker  entoderm- 
cells  '  i ,.  Frequently  this  differentiation  of  the  cell-forms  sets 
in  at  a  verv  early  stage,  during  the  segmentation-process, 
and  is  already  very  appreciable  in  the  blastula. 


'S8 


THE  GASTR.-EA   THEORY 


We  have,  up  to  the  present,  only  considered  that  form  of 
segmentation  and  gastrulation  which,  for  many  and  weighty 
reasons,  we  may  regard  as  the  original,  primordial,  or  palin- 
genetic  form.  We  might  call  it  "  equal  "  or  homogeneous 
segmentation,  because  the  divided  cells  retain  a  resemblance 
to  each  other  at  first  (and  often  until  the  formation  of  the 
blastoderm).  We  give  the  name  of  the  "  bell-gastrula,"  or 
archigastrula,  to  the  gastrula  that  succeeds  it.  In  just  the 
same  form  as  in  the  coral  we  considered  (  monoxenia,  Fig.  31), 
we  find  it  in  the  lowest  zoophyta,  the  gastrophysema  (Fig. 
32),  and  the  simplest  sponges  (olyn- 
thus,  Fig.  38)  ;  also  in  many  of  the 
medusa;  and  hydrapolyps,  lower  types 
of  worms  of  various  classes  (brac- 
hiopod,  arrow-worm,  Fig.  33),  tuni- 
cates  (ascidia,  Plate  XVIII.,  Figs. 
1-4),  many  of  the  echinoderms  (Fig. 
34),  lower  articulates  (Fig.  35),  and 
molluscs  (Fig.  36),  and,  finally,  in  a 
slightly  modified  form,  in  the  lowest 
vertebrate  (the  amphioxus,  Fig.  37  ; 
Plate  XVIII.,  Figs.  5-10). 

The  gastrulation  of  the  amphioxus 
is  especially  interesting  because  this 
lowest  and  oldest  of  all  the  verte- 
brates is  of  the  highest  significance  for 
the  phylogeny  of  the  vertebrate  stem, 
and  therefore  for  our  anthropogeny  (compare  Chapters  XVI. 
and  XVII.),  Just  as  the  comparative  anatomy  of  the  verte- 
brates deduces  the  most  elaborate  features  in  the  structures  of 
the  various  classes  by  divergent  development  from  this  simple 
primitive  vertebrate,  so  comparative  ontogeny  traces  the 
various  secondary  forms  of  vertebrate  gastrulation  to  the 
simple,  primary  formation  of  the  germinal  layers  in  the 
amphioxus.  Although  this  formation,  as  distinguished  from 
the  cenogenetic  modifications  of  the  vertebrate,  may  on  the 
whole  be  regarded  as  palingenetic,  it  is  nevertheless  different 
in  some  features  from  the  quite  primitive  gastrulation  such  as 


Fig.  39.  —  Cells  from 
the  two  primary  ger- 
minal    layers      of     the 

mammal  (from  both  layers  of 
the  blastoderm),  i  larger 
and  darker  cells  of  the  inner 
stratum,  the  vegetal  layer 
or  entoderm,  e  smaller  and 
clearer  cells  from  the  outer 
stratum,  the  animal  layer  or 
ectoderm. 


THE  C.ASTR.EA    THEORY 


we  have,  tor  instance,  in  the  numoxenia  (Fig.  31)  and  t he 
sagitta.  From  Hatschek's  classical  work  (1SS1)  it  is  clear 
that  both  kinds  o(  colls  in  the  germinal  layers  of  the 
amphioxus,  and  many  other  animals,  show  a  diversity  of 
features  very  early  in  the  process  of  segmentation.  Only  the 
first  four  segmentation-cells,  which  are  divided  by  two 
vertical  planes  of  cleavage  cutting-  at  a  right  angle,  are 
homogeneous  (  Plate  XL,  Fig.  8).  The  third,  horizontal  plane 
o\  cleavage  lies,  not  on  the  equator  of  the  ovum,  but  a  little 
above  it,  so  as  to  divide  the  four  blastomeres  into  unequal 
halves — four  smaller  ones  above  and   four  larger  below;   the 


Fig.  40.  Gastrulation  of  the  amphioxus,  from  Hatschek  (vertical 
section  through  the  axis  of  the  ovum).  A,  />',  C  three  stages  in  the  formation 
of  the  blastula  ;  />,  £  curving  of  the  blastula  ;  F complete  gastrula.  /;  segmen- 
tation-cavity,   g  primitive  gfut-cavity. 

former  constitute  the  animal,  and  the  latter  the  vegetal, 
hemisphere.  Hatschek  rightly  observes  that  the  segmenta- 
tion ot  the  ovum  in  the  amphioxus  is  not  strictly  equal,  but 
almost  equal,  and  approaches  the  unequal.  The  difference 
in  size  between  the  two  groups  of  cells  continues  to  be  very 
noticeable  in  the  further  course  of  the  segmentation  ;  the 
smaller  animal  cells  o\  the  upper  hemisphere  divide  more 
quickly  than  the  larger  vegetal  cells  of  the  lower  (Fig.  40 
A,  />).  Hence  the  blastoderm,  which  forms  the  single- 
layer  wall  o\  the  globular  blastula  at  the  end  of  the  cleavage- 
process,  does  not  consist  ot   homogeneous  cells  of  equal  size, 


THE  GASTRMA   THEORY 


as  in  the  sagitta  and  the  monoxenia ;  the  cells  of  the  upper 
half  of  the  blastoderm  (the  mother-cells  of  the  ectoderm)  are 
more  numerous  and  smaller,  and  the  cells  of  the  lower  half 
(the  mother-cells  of  the  entoderm)  less  numerous  and  larger. 
Moreover,  the  segmentation-cavity  of  the  blastula  (Fig.  40 
C,  h)  is  not  quite  globular,  but  forms  a  flattened  spheroid 
with  unequal  poles  of  its  verticle  axis.  While  the  blastula  is 
being  folded  into  a  cup  at  the  vegetal  pole  of  its  axis,  the 
difference  in  the  size  of  the  blastodermic  cells  increases 
(Fig.  40 D,  E);  it  is  most  conspicuous  when  the  invagination 
is  complete  and  the  segmentation- 
cavity  has  disappeared  (Fig.  40  F). 
The  larger  vegetal  cells  of  the  ento- 
derm are  richer  in  granules,  and  so 
darker  than  the  smaller  and  lighter 
animal  cells  of  the  ectoderm. 

But  the  unequal  gastrulation  of  the 
amphioxus  diverges  from  the  typical 
equal  cleavage  of  the  sagitta,  the 
monoxenia  (Fig.  31),  and  the  olyntluts 
(Fig.  38),  not  only  by  this  early  (or 
cenogenetically  premature)  differen- 
tiation of  the  blastodermic  cells,  but 
also  in  another  important  particular. 
The  pure  archigastrula  of  the  latter 
forms  is  uni-axial,  and  it  is  round 
in  its  whole  length  in  transverse  section.  The  vegetal  pole 
of  the  vertical  axis  is  just  in  the  centre  of  the  primitive 
mouth.  This  is  not  the  case  in  the  gastrula  of  the  amphioxus. 
During  the  folding  of  the  blastula  the  ideal  axis  is  already 
bent  on  one  side,  the  growth  of  the  blastoderm  (or  the 
increase  of  its  cells)  being  brisker  on  one  side  than  on  the 
other;  the  side  that  grows  more  quickly,  and  so  is  more 
curved  (Fig.  41  v),  will  be  the  anterior  or  belly-side,  the 
opposite,  flatter  side  will  form  the  back  (d).  The  primitive 
mouth,  which  at  first,  in  the  typical  archigastrula,  lay  at  the 
vegetal  pole  of  the  main  axis,  is  forced  away  to  the  dorsal 
side  ;  and  whereas  its  two  lips  lay  at  first  in  a  plane  at  right 


Fig.   41.  —  Gastrula  of 
the     amphioxus,    seen 

from  left  Side  (diagram- 
matic  median  section). 
(From  Hatschek.)  g  primi- 
tive gut,  u  primitive  mouth, 
p  peristomal  pole-cells,  i 
entoderm,  e  ectoderm,  d 
dorsal  side,  v  ventral  side. 


THE  GASTRJEA   THEORY 


angles  to  the  chief  axis,  they  are  now  so  far  thrust  aside  that 
their  plane  cuts  the  axis  at  a  sharp  angle.  The  dorsal  lip  is 
therefore  the  upper  and  more  forward,  the  ventral  lip  the 
lower  and  hinder.  In  the  latter,  at  the  ventral  passage  of  the 
entoderm  into  the  ectoderm,  there  lie  side  by  side  a  pair  of 
very  large  cells,  one  to  the  right  and  one  to  the  left 
(Fig.  41  p) :  these  arc  the  important  polar  cells  of  the 
primitive  mouth,  or  "the  primitive  cells  of  the  mesoderm." 

In  consequence  of  these  considerable  variations  arising  in 
the  course  of  the  gastrulation,  the  primitive  uni-axial  form 
of  the  archigastrula  in  the  amphioxus  has  already  become 
tri-axial,  and  thus  the  two-sidedness,  or  bilateral  symmetry, 
of  the  vertebrate  body  has  already  been  determined.  The 
vertical  middle  plane  (or  arrow-plane)  passes  between 
the  two  polar  cells  of  the  prostoma,  and  goes  the  whole 
length  of  the  body,  dividing  it  into  two  equal  halves  or 
"antimera,"  right  and  left.  The  primitive  mouth  lies  at  the 
further  and  hinder  end,  a  little  above  the  anti-oral  pole  of  the 
long  axis.  The  arrow-axis,  or  dorso-ventral  axis,  lies  verti- 
cally to  this  chief  axis  on  the  middle  plane,  joining  the  central 
lines  of  the  flat  dorsal  side  and  the  convex  ventral  side.  The 
horizontal  transverse  axis,  or  lateral  axis,  vertical  to  the  two 
(unequally  polar)  axes,  is  equi-polar,  and  crosses  diagonally 
from  right  to  left.  Thus,  the  gastrula  of  the  amphioxus 
alreadv  exhibits  the  characteristic  two-sidedness  of  the  verte- 
brate body,  and  this  has  been  transmitted  from  the  amphioxus 
to  all  the  other  modified  gastrula-forms  of  the  vertebrate  stem. 

Apart    from   this    bilateral    structure,  the   gastrula   o(   the 

amphioxus  resembles  the   typical  archigastrula  of  the   lower 

animals  (Figs.  32   38)  in  developing  the  two  primary  germinal 

layers  from  a  single  layer  of  cells.     This  is  clearly  the  oldest 

and   original    form   of  the    metazoic   embryo.      Although    the 

animals  I  have  mentioned  belong  to  the  most  diverse  classes, 

they    nevertheless   agree  with   each    other,  and    many    more 

animal   forms,    in    having   retained  to   the  present   day,  by  a 

conservative  heredity,  this  palingenetic  form  of  gastrulation 

which  they  have  from   their  earliest  common  ancestors.      Hut 

this   is   not  the  case  with    the  great   majority  of  the  animals. 

M 


162  THE  GASTR.EA    THEORY 

With  these  the  original  embryonic  process  has  been  gradually 
more  or  less  altered  in  the  course  of  millions  of  years  by 
adaptation  to  new  conditions  of  development.  Both  the 
segmentation  of  the  ovum  and  the  subsequent  gastrulation 
have  in  this  way  been  considerably  changed.  In  fact,  these 
variations  have  become  so  great  in  the  course  of  time  that  the 
segmentation  was  not  rightly  understood  in  most  animals, 
and  the  gastrula  was  unrecognised.  It  was  not  until  I  had 
made  an  extensive  comparative  study,  lasting  a  considerable 
time  (in  the  years  1866-75),  in  animals  of  the  most  diverse 
classes,  that  I  succeeded  in  showing  the  same  common 
typical  process  in  these  apparently  very  different  forms  of 
gastrulation,  and  tracing  them  all  to  one  original  form.  I 
regard  all  those  that  diverge  from  the  primary  palingenetic 
gastrulation  as  secondary,  modified,  and  cenogenetic.  The 
more  or  less  divergent  form  of  gastrula  that  is  produced  may 
be  called  a  secondary,  modified  gastrula,  or  a  metagastrula. 

Among  the  many  and  varied  cenogenetic  forms  of 
segmentation  and  gastrulation  I  distinguish  three  chief 
types:  1,  unequal  segmentation  (Plate  II.,  Figs.  7-17); 
2,  discoid  segmentation  (Plate  III.,  Figs.  18-24);  an^  3i  super- 
ficial segmentation  (Plate  III.,  Figs.  25-30).  From  the 
unequal  cleavage  we  have  the  tufted  foetus  ( ainphigaxtru/a, 
Plate  II.,  Figs.  11  and  17);  the  discoid  cleavage  produces 
the  disk-shaped  gastrula  (  discogastntta,  Plate  III.,  Fig.  24)  ; 
and  the  superficial  produces  the  globular  gastrula  ( perigas- 
trula,  Plate  III.,  Fig.  29).  In  the  vertebrates,  with  which  we 
are  chief!)'  concerned,  the  last-named  form  is  not  found  at 
all  ;  on  the  other  hand,  it  is  the  commonest  form  among  the 
articulates  (crabs,  spiders,  insects,  etc.).  Mammals  and 
amphibia  have  the  unequal  segmentation  and  the  tufted 
foetus  ;  so  also  the  ganoid  (scaley)  and  the  round-mouthed 
fishes  (the  lamprey  and  myxine).  On  the  other  hand,  most 
fishes,  and  all  reptiles  and  birds,  have  the  discoid  segmenta- 
tion and  gastrula.     (Cf.  Table  II.,  p.  171.) 

By  far  the  most  important  process  that  determines  the 
various  cenogenetic  forms  of  gastrulation  is  the  change  in  the 
nutrition  of  the  ovum  and  the  accumulation  in  it  of  nutritive 


THE  GASTRASA   THEORY  163 

yelk.  By  this  we  understand  various  chemical  substances 
(chiefly  granules  of  albumin  and  fat-particles)  which  serve 
exclusively  as  reserve-matter  or  food  for  the  embryo.  As  the 
metazoic  embryo  in  its  earlier  stages  of  development  is  not 
yet  able  to  obtain  its  food  and  SO  build  up  the  frame,  the 
necessary  material  has  to  be  stored  up  in  the  ovum.  Hence 
we  distinguish  in  the  ova  two  chief  elements — the  active 
formative  yelk  (protoplasm  or  vitcllus  format 'ivi/s  1  and  the 
passive  food-yelk  (deutoplasm,  or  vitcllus  nutritious,  wrongly 
spoken  oi  as  "  the  yelk,"  lecithus).  In  the  little  palingenetic 
ova,  the  segmentation  of  which  we  have  already  considered, 
the  yelk-granules  are  so  small  and  so  regularly  distributed  in 
the  protoplasm  of  the  ovum  that  the  even  and  repeated 
cleavage  is  not  affected  by  them.  But  in  the  great  majority 
oi  the  animal  ova  the  food-yelk  is  more  or  less  considerable, 
and  is  stored  in  a  certain  part  of  the  ovum,  so  that  even  in  the 
unfertilised  ovum  the  "granary  "  can  clearly  be  distinguished 
from  the  formative  plasm.  As  a  rule,  there  is  then  a  polar 
differentiation  of  the  ovum,  in  the  sense  that  a  chief  axis  can 
be  discerned  in  it,  and  the  formative  yelk  (with  the  germinal 
vesicle)  gathers  at  one  pole  and  food-yelk  at  the  other.  The 
first  is  the  animal,  and  the  second  the  vegetal,  pole  of  the 
vertical  axis  of  the  ovum. 

In  these  "  telolecithal "  ova  (for  instance,  in  the  cyclos- 
toma  and  amphibia,  Plate  II.,  Figs.  7-11)  the  gastrulation  then 
usually  takes  place  in  such  a  way  that  in  the  cleavage  of  the 
impregnated  ovum  the  animal  (usually  the  upper)  half  splits 
up  more  quickly  than  the  vegetal  (lower).  The  contractions 
oi  the  active  protoplasm,  which  effect  this  continual  cleavage 
oi  the  cells,  meet  a  greater  resistance  in  the  lower  vegetal 
half  from  the  passive  deutoplasm  than  in  the  upper  animal 
half.  Hence  we  find  in  the  latter  more  but  smaller,  and  in 
the  former  fewer  but  larger,  cells.  The  animal  cells  produce 
the  external,  and  the  vegetal  cells  the  internal,  germinal 
layer. 

Although  this  unequal  segmentation  of  the  cyclostoma, 
ganoids,  and  amphibia  seems  at  first  sight  to  differ  from  the 
original  equal  segmentation  (for   instance,  in    the    monoxenia, 


1 64  THE  GASTK.EA    THEORY 

Fig.  31),  they  both  have  this  in  common,  that  the  cleavage 
process  throughout  affects  the  whole  cell  ;  hence  Remak 
called  it  total  segmentation,  and  the  ova  in  question  holoblastic. 
It  is  otherwise  with  the  second  chief  group  of  ova,  which  he 
distinguished  from  these  as  meroblastic :  to  this  class  belong 
the  familiar  large  eggs  of  birds  and  reptiles,  and  of  most 
fishes.  The  inert  mass  of  the  passive  food-yelk  is  so  large 
in  these  cases  that  the  protoplasmic  contractions  of  the  active 
yelk  cannot  effect  any  further  cleavage.  In  consequence, 
there  is  only  a  partial  segmentation.  While  the  protoplasm 
in  the  animal  section  of  the  ovum  continues  briskly  to  divide, 
multiplying  the  nuclei,  the  deutoplasm  in  the  vegetal  section 
remains  more  or  less  undivided  ;  it  is  merely  consumed  as 
food  by  the  forming  cells.  The  larger  the  accumulation  of 
food,  the  more  restricted  is  the  process  of  segmentation.  It 
may,  however,  continue  for  some  time  (even  after  the  gastru- 
lation  is  more  or  less  complete)  in  the  sense  that  the  vegetal 
cell-nuclei  distributed  in  the  deutoplasm  slowly  increase  by 
cleavage  ;  as  each  of  them  is  surrounded  by  a  small  quantity 
of  protoplasm,  it  may  afterwards  appropriate  a  portion  of  the 
food-yelk,  and  thus  form  a  real  "  yelk-cell  "  (  merocyte  I.  When 
this  vegetal  cell-formation  continues  for  a  long  time,  after 
the  two  primary  germinal  layers  have  been  formed,  it  takes 
the  name  of  the  "after-segmentation  "  (Waldeyer). 

The  meroblastic  ova  (Plate  III.)  are  only  found  in  the 
larger  and  more  highly  developed  animals,  and  only  in  those 
whose  embryo  needs  a  longer  time  and  richer  nourishment 
within  the  foetal  membranes.  According  as  the  yelk-food 
accumulates  at  the  centre  or  the  side  of  the  ovum,  we  dis- 
tinguish two  groups  of  dividing  ova,  periblastic  and  dis- 
coblastic.  In  the  periblastic  the  food-yelk  is  in  the  centre, 
enclosed  inside  the  ovum  (hence  they  are  also  called  "centro- 
lecithal  "  ova)  :  the  formative  yelk  surrounds  the  food-yelk, 
and  so  suffers  itself  a  superficial  cleavage.  This  is  found 
among  the  articulates  (crabs,  spiders,  insects,  etc.,  Plate  III., 
Figs.  25-30).  In  the  discoblastic  ova  the  food-yelk  gathers 
at  one  side,  at  the  vegetal  or  lower  pole  of  the  vertical  axis, 
while  the  nucleus  of  the  ovum  and  the  great  bulk  of  the 


THE  CASTh'.EA    THEOR\  165 


formative  yolk  lie  at  the  upper  or  animal  pole  (hence  these 
ova  are  also  called  "  tclolethical  ").  In  these  cases  the 
Cleavage  of  the  ovum  begins  at  the  upper  pole,  and  leads  to 
the  formation  of  a  dorsal  discoid  embryo.  This  is  the  case 
with  all  meroblastic  vertebrates,  most  fishes,  the  reptiles  and 
birds,  and  the  oviparous  mammals  (monotrema). 

The  gastrulation  of  the  discoblastic  ova,  which  chiefly 
concerns  us,  otters  serious  difficulties  to  microscopic  investi- 
gation and  philosophic  consideration.  These,  however,  have 
been  mastered  by  the  comparative  embryological  research 
which  has  been  conducted  by  a  number  of  distinguished 
observers  during  the  last  few  decades — especially  the  brothers 
Hertwig,  Rabl,  Kupffer,  Selenka,  Riickert,  Goette,  Rauber, 
etc.  These  thorough  and  careful  studies,  aided  by  the  most 
perfect  modern  improvements  in  technical  method  (in  tinting 
and  dissection),  have  given  a  very  welcome  support  to  the 
views  which  I  put  forward  in  my  work,  On  /lie  Gastrula  and 
the  Segmentation  of  the  Animal  Ovum  |not  translated],  in 
1875.  As  it  is  very  important  to  understand  these  views  and 
their  phylogenetic  foundation  clearly,  not  only  as  regards 
evolution  in  general,  but  particularly  in  connection  with  the 
genesis  of  man,  I  will  give  here  a  brief  statement  o(  them  as 
far  as  they  concern  the  vertebrate-stem  : — 

1.  All  the  vertebrates,  including  man,  are  phylogenetically 
(or  genealogically)  related — that  is,  are  members  of  one 
single  natural  stem. 

2.  Consequentlv,  the  embryonic  features  in  their  indi- 
vidual development  must  also  hang  together  phylogenetically. 

3.  As  the  gastrulation  of  the  amphioxus  shows  the  original 
palingenetic  form  in  its  simplest  features,  that  of  the  other 
vertebrates  must  have  been  derived  from  it. 

4.  The  eenogenetic  modifications  of  the  latter  are 
more  appreciable  the  more  food-yelk  is  stored  up  in  the 
ovum. 

5.  Although  the  mass  of  the  food-yelk  may  be  very  large 
in  the  ova  of  the  discoblastic  vertebrates,  nevertheless  in 
every  case  a  blastula  is  developed  from  the  morula,  as  in  the 
holoblastic  ova. 


166  THE  GASTRASA  THEORY 

6.  Also,  in  every  case,  the  gastrula  developes  from  the 
blastula  by  folding,  or  invagination. 

7.  The  cavity  which  is  produced  in  the  foetus  by  this 
folding  is,  in  each  case,  the  primitive  gut  (progaster),  and  its 
opening  the  primitive  mouth  (prostoma). 

8.  The  food-yelk,  whether  large  or  small,  is  always  stored 
in  the  ventral  wall  of  the  primitive  gut ;  the  cells  (called 
"merocvtes")  which  may  be  formed  in  it  subsequently  (bv 
"  after-segmentation ")  also  belong  to  the  inner  germinal 
layer  or  endoblast,  like  the  cells  which  immediately  enclose 
the  primitive  gut-cavity. 

9.  The  primitive  mouth,  which  at  first  lies  below  at  the 
basic  pole  of  the  vertical  axis,  is  forced,  by  the  growth  of  the 
yelk,  backwards  and  then  upwards,  towards  the  dorsal  side  of 
the  embryo  ;  the  vertical  axis  of  the  primitive  gut  is  thus 
gradually  converted  into  horizontal. 

10.  The  primitive  mouth  is  closed  sooner  or  later  in  all 
the  vertebrates,  and  does  not  pass  into  the  permanent  mouth- 
aperture;  it  rather  corresponds  to  the  "properistoma,"  or  region 
of  the  anus.  From  this  important  point  the  formation  of  the 
middle  germinal  layer  proceeds,  between  the  two  primary 
layers. 

The  wide  comparative  studies  of  the  scientists  I  have 
named  have  further  shown  that  in  the  case  of  the  discoblastic 
higher  vertebrates  (the  three  classes  of  amniotes)  the  primi- 
tive mouth  of  the  embryonic  disc,  which  was  long  looked  for 
in  vain,  is  found  always,  and  is  nothing  else  than  the  familiar 
"  primitive  groove."  This  is  a  groove  that  lies  in  the  hinder 
dorsal  surface  of  the  discoid  gastrula,  and  was  formerly  con- 
fused with  the  hinder  part  of  the  medullary  tube.  It  is  true 
that  it  is  directly  connected  with  this  for  some  time  (bv  the 
canalis  neurentericus,  which  we  shall  discuss  later),  but 
originally  it  is  a  totally  different  thing,  both  in  structure  and 
purport.  The  two  parallel  longitudinal  swellings  which 
enclose  this  slender  "  primitive  groove"  (lying  on  the  middle 
line)  are  the  right  and  left  primitive  lips.  The  primitive 
mouth,  which  is  at  first  (in  the  holoblastic  vertebrates)  a  small 
round  opening,  is  thus  altered  (in  consequence  of  the  increasing 


THE  GASTRJEA   THEORY  167 

accumulation  of  food-yelk  and  the  resulting  extension  of  the 
ventral  wall  oi  the  primitive  gut)  not  only  in  position  and 
direction,  but  also  in  shape  and  size.  It  changes  first  into  a 
sickle-shaped  tranverse  told  (the  "  sickle-groove "),  in  which 
we  distinguish  a  ventral  (lower)  and  a  dorsal  (upper)  primitive 
lip.  However,  the  broad  transverse  fold  soon  narrows,  and 
changes  into  a  longitudinal  fold  (something  like  a  hare-slit), 
the  right  and  left  halves  of  the  sickle-groove  (called  the 
"sickle-horns")  being  shortened,  the  middle  part  and  the  two 
halves  of  the  dorsal  upper  lip  being  drawn  forward.  The 
latter  meet  subsequently  in  the  middle  line,  and  form  the 
important  "  primitive  streak." 

Thus  gastrulation  may  be  reduced  to  one  and  the  same 
process  in  all  the  vertebrates.  Moreover,  the  various  forms 
it  takes  in  the  invertebrate  metazoa  can  always  be  reduced  to 
one  of  the  four  types  of  segmentation  described  above.  In 
relation  to  the  distinction  between  total  and  partial  segmenta- 
tion, the  grouping  of  the  various  forms  is  as  follows  : — 

I.     Palinerenetic  (         r-       ,  -, 

1.   Equal  segmentation  1     ,      t  .   , 

primitive  ,  ',,    I,         ,     1    ,  A.      tola    segmenta- 

1  beu-erastrula). 
segmentation.         t  I  tion 

il«  ithout  indepen- 
dent food-yelk), 

( enogenetic  v 


""SSI    "  '    3-   Discoid  segmentation         ,     ,       p      ^ 

(modified  by  (discoid  gastrula).  |  ^.^ 

adaptation).  c  c    ■    ,  ,    ,•  I  (with  indepen- 


c         .-   .   .  with  indepen- 

4.  Super lii-ial  segmentation  ,        ,      ,   '  ..  , 

^           ,11.1,  tli.-iit  lood-yelkl. 

(spherical  gastrula).  J 


The  lowest  metazoa  we  know — namely,  the  lower  zoophyta 
(sponges,  simple  polyps,  etc.) — remain  throughout  life  at  a 
stage  o\  development  which  differs  little  from  the  gastrula  ; 
their  whole  body  consists  of  two  layers  of  cells.  This  is  a 
fact  o(  extreme  importance.  We  see  that  man,  and  even 
other  vertebrates,  pass  quickly  through  a  stage  o(  develop- 
ment in  which  they  consist  oi  two  layers,  just  as  these  lower 
zoophyta  do  throughout  life.  If  we  apply  our  biogenetic 
law  to  the  matter,  we  at  once  reach  this  important  conclusion  : 
"  Man  and  all  the  other  animals  which  pass  through  the  two- 
layer  stage,  or  gastrula-form,  in  the  course  of  their  embryonic 


THE  GASTR.EA    THEORY 


development,  must  descend  from  a  primitive  simple  stem-form, 
the  whole  body  of  which  consisted  throughout  life  (as  is  the 
case  with  the  lower  zoophyta  to-dav)  merely  of  two  cell-strata 
or  germinal  lavers."  We  will  call  this  primitive  stem-form, 
with  which  we  shall  deal  more  fully  later  on,  the  gastrcea — 
that  is  to  sav,  "  primitive-gut  animal." 

According  to  this  gastraea  theory,  one  organ  was  originally 
of  the  same  morphological  and  physiological  significance  in 
all  multicellular  animals — the  primitive  gut  ;  and  the  two 
primarv  germinal  layers  which  form  its  wall  must  also  be 
regarded  as  similar  or  homologous  in  all.  This  important 
homology  of  the  primary  germinal  layers  is  proved,  on  the 
one  hand,  from  the  fact  that  the  gastrula  was  originally 
formed  in  the  same  way  in  all  cases — namely,  by  the  folding 
of  the  blastula;  and,  on  the  other  hand,  by  the  fact  that  in 
everv  case  the  same  fundamental  organs  arise  from  the 
germinal  layers.  The  outer  or  animal  layer,  or  ectoderm, 
always  forms  the  chief  organs  of  animal  life — the  skin, 
nervous  system,  sense-organs,  etc.  ;  the  inner  or  vegetal 
layer,  or  entoderm,  gives  rise  to  the  chief  organs  of  vegetative 
life — the  organs  of  nourishment,  digestion,  blood-forma- 
tion, etc. 

In  the  lower  zoophvta,  whose  bodv  remains  at  the  two- 
layer  stage  throughout  life,  the  gastrajada,  the  simplest 
sponges  i  olynthus  j,  and  polyps  ( hydra  J,  these  two  groups 
of  functions,  animal  and  vegetative,  are  strictly  divided 
between  the  two  simple  primary  layers.  Throughout  life  the 
outer  or  animal  blastodermic  layer  acts  simply  as  a  covering 
for  the  body,  and  accomplishes  its  movement  and  sensation. 
The  inner  or  vegetative  laver  of  cells  acts  throughout  life  as  a 
gut-epithelium,  or  nutritive  laver  of  enteric  cells,  and  often 
also  releases  the  reproductive  cells. 

The  best  known  of  these  "  gastrasada,"  or  "  gastrula-like 
animals,"  is  the  common  fresh-water  polyp  (hydra).  This 
simplest  of  all  the  cnidaria  has,  it  is  true,  a  crown  of  tentacles 
round  its  mouth.  Also  its  outer  germinal  layer  is  slightly 
differentiated  histologically.  But  these  are  secondary  addi- 
tions, and    the    inner  germinal   laver  is  a  simple   stratum  of 


THE  GASTRJEA   THEORY  169 

colls.     On  the  whole,  the  hydra  lias  preserved  to  our  day  by 

heredity   the   simple  structure  o(   our  primitive  ancestor,  the 
gastrcea  (cf.  Chapter  XIX.). 

In  all  other  animals,  particularly  the  vertebrates,  the 
gastrula  is  merely  a  brief  transitional  stage.  Here  the  two- 
layer  Stage  of  the  embryonic  development  is  quickly  succeeded 
by  a  three-layer,  and  then  a  four-layer,  stage.  With  the 
appearance  of  the  four  superimposed  germinal  layers  we  reach 
again  a  firm  and  steady  standing-ground,  from  which  we  may 
follow  the  further,  and  much  more  difficult  and  complicated, 
course  of  embryonic  development. 


EXPLANATION   OF    PLATES    II.  AND    III. 
Segmentation  and  Gastrulation. 

Plates  II.  and  III.  illustrate  the  chief  differences  in  the  ovum-segmentation 
and  gastrulation  of  animals  by  diagrammatic  sections.  Plate  II.  shows 
holoblastic  ova  (with  total  segmentation) ;  Plate  III.,  meroblastic  ova  (or  with 
partial  segmentation).  The  animal  half  of  the  ova  (ectoderm)  is  tinted  grey, 
and  the  vegetal  half  (entoderm  with  food-yelk)  red.  The  food-yelk  is  vertically 
grained.  All  sections  are  vertical  and  median  (through  the  axis  of  the  primi- 
tive gut).  The  letters  have  the  same  meaning  throughout:  c  Stem-cell 
( ' cytula ).  f  Segmentation-cells  ( segmentella  or  blastomeres ).  m  Mulberry- 
stage  (morula),  b  Blastula.  g  Cup-structure  (gastrula).  s  Segmentation- 
cavity  (blastoccelum),  d  Primitive  gut-cavity  (progasier).  u  Primitive  mouth 
( prostoma ).  n  food-yelk  (lecithus).  i  gut-layer  ( entodcrma ).  e  skin-layer 
(  ectoderma  ). 

Figs.  1-6.  Equal  Segmentation  of  a  lower  metazoon  ^///ii,  ascidia). 
Fig.  r.  Stem-cell  (cytula).  Fig.  2.  Cleavage-stage  with  four  segmentation- 
cells.  Fig.  3.  Mulberry-stage  (morula).  Fig.  4.  Blastula.  Fig.  5.  The  same 
in  process  of  folding  or  invagination  (depula).  Fig.  6.  Bell-gastrula  (archi- 
gastrula).      Cf.  Figs.  31-40. 

Figs.  7-1 1.  Unequal  Segmentation  of  an  amphibian  (frog).  Fig.  7. 
Stem-cell.  Fig.  8.  Cleavage  stage  with  four  segmentation-cells.  Fig.  9. 
Morula.     Fig.   10.  Blastula.      Fig.  11.  Tuft-gastrula  (amphigastrula).      Cf.  Figs. 

-+2-5.3- 

Figs.  12-17.  Unequal  segmentation  of  a  mammal  (hare).  Fig.  12. 
Cytula.  Fig.  13.  Cleavage  with  two  segmentation-cells  (e  mother-cell  of  the 
ectoderm,  i  mother-cell  of  the  entoderm).  Fig.  14.  Cleavage-stage  with  four 
segmentation-cells.  Fig.  15.  Beginning  of  the  folding  of  the  blastula.  Fig. 
16.  Progress  of  the  invagination.  Fig.  17.  Tufted  gastrula  (amphigastrula), 
Cf.  Figs.  6b-75. 

Figs.  18-24.  Discoid  segmentation  of  a  bony  fish  (labrus?  cottus?). 
Most  of  the  food-yelk  (  n  )  is  left  out  (cf.  Figs.  60-65).  Fi»-  lS-  Cytula.  Fig. 
19.  Cleavage-stage  with  two  cells.  Fig.  20.  Cleavage-stage  with  thirty-two 
cells.  Fig.  21.  Mulberry-stage  (morula).  Fig.  22.  Blastula.  Fig.  23.  The 
same  in  process  of  invagination  (depula).  Fig.  24.  Discoid  gastrula  (disco- 
gastrula). 

Figs.  25-30.    Superficial  segmentation  of  a  crab  (peneus).    Fig.  25. 

Cytula.  Fig.  26.  Cleavage-stage  with  eight  cells  (only  four  visible).  Fig.  27. 
Cleavage  stage  with  thirty-two  cells.  Fig.  28.  Morula  and  blastula.  Fig,  29, 
Spherical  gastrula  (perigastrula).  Fig.  30.  Passage  of  the  gastrula  into  the 
nauplius  embryo  :  the  gullet-cavity  has  been  formed  in  front  of  the  primitive 
gut  by  folding  from  without. 

(Cf.  the  following  Tables  II.-III.) 


Gastrulation. 


PI  R 


VI. 


A 


if). 


'•' 

$5& 

0 

&r 

11. 

1 

0 

p 

g    1 

Won 


h  roo 


Mammal 


Gastrulation. 


Pi. 


18 


<&¥ 


Fish 


SECOND  TABLE 

SUMMARY    OF   THE   CHIEF   DIFFERENCES    IX 

THE   OVUM-SEGMENTATION    AND   GASTRU- 

LATION    OF    ANIMALS 

Tin' animal  stems  arc  indicated  by  the  letters  a  g:  n  Zoophyta.     b  Annelida. 
c  Mollusca.     d  Echinoderma.     e  Articulata.    f  Tunicata.    g  Vertebrata. 


I. 

Total 

Segmentation. 

Holoblastic 


Gastrula 
without 
separate 

food-yelk. 

I  lologastrula. 


I.  Primitive 
segmentation. 
Archiblastic  ova. 

Bell-gastrula 

(archigastrula). 

Plate  II.,  Fijfs.  1-6. 


- 


II.  Unequal 
Segmentation. 
Amphiblastic  ova. 

Tufted-gastrula 

(amphigastrula  i. 

Plate  II..  Fies.  7   17. 


a.  Many  lower  zoophyta  (sponges, 
hydrapolyps,  medusae,  simpler 
corals). 

b.  Many  lower  annelids  (sagitta, 
phoronis,  many  nematoda, etc. , 
terebratula,  argiope,  pisidium). 

r.     Some  lower  molluscs. 

(/.   Many  echinoderms. 

e.  A  few  lower  articulata  (some 
branchiopods,  copepods:  Tar- 
digrades,  pteromalina ). 

f.  Many  tunicata. 

g.  The  acrania  (amphioxus). 


11.  Many  zoophyta  (sponges, 
medusae,  corals,  siphonophora, 
ctenophora). 

/>.   Most  worms. 

r.    Most  molluscs. 

(/.  Many  echinoderms  (viviparous 
species  and  some  others). 

e.  Some  of  the   lower  articulata 

(both  Crustacea  and  tracheal.!  |. 

f.  Many  tunicata. 

g.  Cyclostoma,  the  oldest  fishes, 
amphibia,  mammals  1  not  includ- 
ing man). 


II. 

Partial 

segmentation. 

Meroblastic 

ova. 


Gastrula  with 

separate 

food-yelk. 

Merogastrula. 


III.  Discoid 
Segmentation. 
Discoblastic  ova. 

Discoid  gastrula. 

Plate  III.,  Figs,    is   2+ 


IV.  Superficial 
Segmentation. 

Periblast  ic  ova. 

Spherical-gas- 
trula. 

Plate  III.,   Figs.  2S   30. 


Cephalopods  or  cuttle-fish. 
Many     articulata,     wood-lie 

scorpions,  etc. 

Primitive     lishes.     bony    lishe 

reptiles,  birds,  monot  i  ernes. 


Tlu-  great  majority  of  the  arli- 
t  usi  iceans.iin  riapods, 
arachnids,  insi 


THIRD  TABLE 

SUMMARY    OF  THE    FIRST    FOUR    EMBRYONAL 

STAGES  IN  ANIMALS  IN  RELATION  TO  THE 

FOUR  CHIEF  FORMS  OF  SEGMENTATION 


A.   Total  Segmentation. 


a.  Original  or 

Primordial 
Segmentation. 


b.   Unequal 
Segmentation. 


B.   Partial  Segmentation. 


d.    Superficial 
Segmentation. 


Examples  : 
Monoxenia, 

Sagitta, 
Amphioxus. 

la.  Arehieytula, 

Archiblastic 

stem-cell 

(Plate  II.,  Fit;-,  i). 

A  single  cell,  in 

which        formative 

yelk  and  food-yelk 

are  not  separated. 


I  la.  Arehimorula 

(Plate  II.,  Fig.  3). 
A     solid,     gene- 
rally globular,  clus- 
ter of  homogeneous 

cells. 


Ilia.  Arehi- 
blastula 

(Plate  II.,  Fig.  4). 
A  hollow  (gene- 
rail  y  g  lobular) 
sphere,  the  wall 
consisting  of  a  sin- 
gle layer  of  homo- 
geneous cells. 


IVa.  Arehi- 

gastrula, 

Bell-gastrula 

(Plate  II.,  Fig.  6). 
F'gs.  32-3S. 
P  r  i  111  i  t  i  v  e  g  u  t 
empty,  "  wit  hout 
food-yelk.  Primary 
germinal  layers  of 
one  stratum. 


Examples  : 
Cyclostoma, 

Amphibia, 
Mammals. 

lb.  Amphieytula, 

Amphiblastie 

stem-cell 

(Plate  II., Figs.  7,  iz). 

A  uni-axial  cell, 
containing-  forma- 
tive yelk  at  the 
animal  pole  and 
food-yelk  at  the 
vegetal  pole,  not 
clearly  separated. 


llb.Amphimorula 

(Plate  II.,  Fig.  9). 
A  roundish  clus- 
ter of  two  kinds  of 
cells,  the  smaller 
at  the  animal  and 
the  larger  at  the 
vegetal  pole. 


nib.  Amphi- 
blastula 

(Plate  II.,  Fig.  10). 
A  roundish  sphere, 
the  wall  consisting 
of  small  cells  at 
the  animal  and 
large  cells  at  the 
vegetal  pole. 


I\'b.  Amphi- 

gastrula, 
Tufted-gastrula 

(Plate  II.,  Figs,  n, 
17)- 
Fig-  So- 
Primitive       gut 
partly    filled    with 
divided  food-yelk. 
Germinal  layers  of 
several  strata. 


Examples  : 

Fishes, 

Reptiles, 

Birds. 

Ic.  Diseoeytula, 

Discoblastic 
stem-cell 
(Plate  III.,  Fig.  lS). 
A  very  large  uni- 
axial cell,  contain- 
ing formative  yelk 
at  the  animal  pole 
and  food-yelk  at 
the  vegetal,  the 
two  clearly  sepa- 
rated. 

lie.  Diseomorula 

(Plate  III.,  Fig.  zi). 
A  flat  disc,  con- 
sisting of  homo- 
geneous cells  at 
the  animal  pole  of 
the  food-velk. 


Examples  : 
Crustacea, 

Arachnida, 
Insects. 

id.  Perieytula, 

Periblast  ic 
stem-cell 
(Plate  III.,  Fig.  25). 
A  large  cell,  con- 
taining formative 
yelk  at  the  peri- 
phery, and  food- 
velk  in  the  centre. 


1  id.  Perimorula 

(Plate  III.,  Fig.  27). 
A  closed  sphere  ; 
one  layer  of  cells 
encloses  the  whole 
of  the  central  food- 
yelk,  which  con- 
tains dividing  nu- 
clei. 


nic.  Diseo-  Hid.  Peri- 

blastula  blastula 

(Plate  III.,  Fig.  22).(Plate  III.,  Fig.  jS). 
Aroundishsphere,  A  closed  sphere  ; 
the  smaller  liemi-  one  layer  of  cells 
sphere  consisting  encloses  the  whole 
of  segmentation-  of  the  central  food- 
cells  and  the  larger  yelk  ;  all  the  nuclei 
of  food-velk.  have  been    driven 

to  the  surface. 


IVc.  Diseo- 
gastrula, 


IVd.  Peri- 
gastrula, 


Disc-gastrula  Spherical-gastrula 
(Plate  III.,  Fig.  J4 ).  (Plate  III.,  Fig.  29). 
Figs.  62-65.  Segmentation- 
Primitive  gut  cavity  full  of  undi- 
filled  with  undi-  vided  food  -  yelk, 
vided  food -yelk.  Primitive  gut  su- 
Flat  germinal  disc,  perficial. 


FOURTH   TABLE 

SUMMARY    OF   THE   CHIEF   VARIATIONS    l\ 
llll".    RHYTHM    OF     THE    OVUM-SEGMENTATION 

(Only  the  tir>t  row  [Sagitta]  shows  the  original  palingenetic   rhythm  of  the 
segmentation  in  regular  geometrical  progression.      All  the  other  rows  show 
dary,      cenogenetic      modifications,     c—  Stem-cell,     s  =  Segmentation- 
cells,     e      Ectoderm-cells,     i    :  Entoderm-cells.) 


I. 

,,. 

III. 

IV. 

Y. 

VI. 

Arrow- 

Amphibian 

Mammal 

Snail 

Worm 

Worm 

worm 
(Sagitta) 

(Frog) 

(Hare) 

(Troehus) 

(Fabricia) 

(Cyglo- 
gena) 

1. 

\< 

If 

If 

If 

If 

2s 

2  s 

2  s 
(1  e      1 «) 

2  s 

2s 
(1  e  4   it) 

2  s 
(IM   If) 

4rS 

4s 

4s 

4  s 

3  s 

3  s 

(2,-+    2f) 

(2 «       if) 

(2*—  1  f) 

8  s 

as 

8s 

8  s 

5  s 

4s 

(4«  +  4») 

•4''       4'") 

(4«  +  4«) 

(4«+i  0 

(3«       .  1 

12  x 

12  s 

12  s 

6  s 

5  s 

(8«      4.-' 

(8«       4'l 

(8e  +  4«) 

i4.-      21) 

(4<^+   "') 

16  s 

16  s 

16  s 

20  s 

10  s 

6  s 

IS,-     -  S/l 

(8«      8») 

(8«4-2f) 

(S<      if) 

24  s 

24  s 

24  s 

lis 

7  s 

(i6e  +  8«) 

(16  e  +8/) 

(i6<?4-  81) 

(8«  +  3f) 

(6i>  4-  if) 

32  s 

32  v 

32  s 

40  s 

19  s 

8  s 

(i6<?  +  16  ») 

(l6<>4    16  i) 

(32  e+  8i) 

(i6«      3O 

(7.'       if) 

48  5 

48  s 

44 

21s 

9  s 

(32 '+  '6') 

(32      t6«) 

(3»+  '-'l 

5«) 

(8c      if) 

64  s 

64  s 

64  s 

76  s 

37  s 

10  s 

{32  e  +  32  i) 

(32  t         32  f) 

(32 1      32 1) 

(64 e  -    12 1) 

1.5-  <■      s«) 

ige  +  1  / ) 

96  s 

96  s 

84  s 

38  s 

(64  e      32.) 

(64  (  +  20 «) 

128^ 

160  $ 

148  s 

70s 

(64 «      61) 

CHAPTER  IX. 

THE  GASTRULATION  OF  THE  VERTEBRATE' 

Phylogenetic  unity  of  the  vertebrate-stem.  Ontogenetic  unity  of  its  gastrula- 
tion.  Historical  relations  of  holoblastic  and  meroblastic  vertebrates. 
Unequal  segmentation  of  the  ovum  and  amphigastrula  of  the  amphibia 
(tailless  frogs  and  tailed  salamanders).  Their  segmentation-cavity  (blas- 
tocoel)  and  primitive-gut  cavity  (Rusconic  gastric  cavity).  Derivation 
of  partial  from  total  segmentation.  Discoblastic  vertebrates,  with  germinal 
disc  (discoid  gastrula).  Deep-sea  bony  fishes  with  small  and  shark 
with  large  food-yelk.  Epigastrula  (or  narrow-mouthed  discoid  gastrula) 
of  the  amniota.  The  hen's  <^^g  and  its  large  food-yelk.  Discoid  gastrula- 
tion  of  the  sauropsida  (reptiles  and  birds)  and  monotrema.  The  primitive 
groove  of  the  amniote-embryo  is  the  primitive  mouth  of  their  discoid 
gastrula.  Phylogenetic  disappearance  of  the  food-yelk  in  the  mammal. 
Oviparous  and  viviparous  mammals.  Gastrulation  of  the  opossum  and  the 
hare.      Superficial  segmentation  of  the  articulata. 

The  remarkable  processes  of  gastrulation,  ovum-segmenta- 
tion, and  formation  of  germinal  layers  present  a  most  con- 
spicuous variety.  There  is  to-day  only  the  lowest  of  the 
vertebrates,  the  amphioxus,  that  exhibits  the  original  form 
of  those  processes,  or  the  palingenetic  gastrulation  which  we 
have  considered  in  the  preceding  Chapter,  and  which  culmi- 
nates in  the  formation  of  the  archigastrula  (Fig.  40).  In  all 
other  extant  vertebrates  these  fundamental  processes  have 
been  more  or  less  modified  by  adaptation  to  the  conditions  of 
embryonic  development  (especially  by  changes  in  the  food- 
yelk)  ;  they  exhibit  various  cenogenetic  forms  of  the  formation 
of  germinal  layers,  and  thus  develop  by  means  of  a  meta- 
gastrula.  However,  the  different  classes  vary  considerably 
from  each  other.  In  order  to  grasp  the  unity  that  underlies 
the  manifold  differences  in  these  phenomena  and  their  his- 
torical connection,  it  is  necessary  to  bear  in  mind  always  the 
unity  of  the  vertebrate-stem.  This  "phylogenetic  unity," 
which  I  systematically  developed  in  my  Generelle  Morphologie 

1  Cf.    Balfour's    Manual  of  Comparative    Embryology,    vol.     ii.;     Theodore 
Morgan's  The  Development  of  the  Frog's  Egg. 
'74 


THE  GASTRULATION  OF  THE  VERTEBRATE  175 

in  1866,  is  now  generally  admitted.  All  impartial  zoologists 
agree  to-day  that  all  the  vertebrates,  from  the  amphioxus  and 
the  fishes  to  the  ape  and  man,  descend  from  a  common 
ancestor,  "the  primitive  vertebrate."  Hence  the  ontoge- 
netic processes,  by  which  each  individual  vertebrate  is 
developed,  must  also  be  capable  of  being  reduced  to  one 
common  type  ol  embryonic  development ;  and  this  primitive 
type  is  most  certainly  exhibited  to-day  by  the  amphioxus. 

It  must,  therefore,  be  our  next  task  to  make  a  comparative 
study  of  the  various  forms  of  vertebrate  gastrulation,  and 
trace  them  phylogenetically  to  that  of  the  lancelet.  Broadly 
speaking,  they  fall  first  into  two  groups  :  the  older  cyclos- 
toma,  the  earliest  fishes,  most  of  the  amphibia,  and  the 
viviparous  mammals,  have  holoblastic  ova  with  total,  unequal 
segmentation  ;  while  the  younger  cyclostoma,  most  of  the 
fishes,  ccecilia,  reptiles,  birds,  and  monotrema,  have  mcro- 
blastic  ova,  with  partial  discoid  segmentation.  A  closer 
study  oi  them  shows,  however,  that  these  two  groups  do  not 
present  a  natural  unity,  and  that  the  historical  relations 
between  their  several  divisions  are  very  complicated.  In 
order  to  understand  them  properly,  we  must  first  consider  the 
various  modifications  of  gastrulation  in  these  classes.  We 
may  begin  with  that  of  the  amphibia. 

The  most  suitable  and  most  available  object  of  study  in 
this  class  are  the  eggs  of  our  indigenous  amphibia,  the 
tailless  frogs  and  toads,  and  the  tailed  salamander.  In  spring 
they  are  to  be  found  in  clusters  in  every  pond,  and  careful 
examination  of  the  ova  with  a  lens  is  sufficient  to  show  at 
least  the  external  features  of  the  segmentation.  In  order  to 
understand  the  whole  process  rightly  and  follow  the  forma- 
tion of  the  germinal  layers  and  the  gastrula,  the  ova  of  the 
frog  and  salamander  must  be  carefully  hardened  ;  then  the 
thinnest  possible  sections  must  be  made  of  the  hardened  ova 
with  the  microtome,  and  the  tinted  sections  must  be  very 
closely  compared  under  a  powerful  microscope. 

The  ova  of  the  frog  or  toad  are  globular  in  shape,  about 
two  millimetres  in  diameter,  and  are  clustered  in  jelly- 
like   masses,    which    are     lumped    together    in    the    case    of 


176 


THE  GASTRULATION  OF  THE   VERTEBRATE 


the  frog,  but  form  long  strings  in  the  case  of  the  toad. 
When  we  examine  the  opaque,  grey,  brown,  or  blackish  ova 
closely,  we  find  that  the  upper  half  is  darker  than  the  lower. 
The  middle  of  the  upper  half  is  in  many  species  black,  while 
the  middle  of  the  lower  half  is  white.1  In  this  way  we  get 
a  definite  axis  of  the  ovum  with  two  poles.     To  give  a  clear 


Fig.  42.— The    Cleavage    Of    the    frog's    OVUm    (magnified   ten   times), 
-i  stem-cell.     B  the  first  two  segmentation-cells.     C  four  cells.     D  eight  cells 

(4  animal  and  4  vegetative).  E  twelve  cells  (8  animal  and  4  vegetative). 
/"sixteen  cells  (8  animal  and  8  vegetative).  G  twenty-tour  cells  (10  animal 
and  8  vegetative).  H  thirty-two  cells.  /  forty-eight  cells.  A'  sixty-tour  cells. 
L  ninety-six  cells.      .1/  160  cells  ( 128  animal  and  ,}2  vegetative). 

idea  of  the  segmentation  of  this  ovum,  it  is  best  to  compare 
it  with  a  globe  on  the  surface  of  which  are  marked  the  various 
parallels  of  longitude  and  latitude.     The  superficial  dividing 


1  The  colouring  of  the  eggs  of  the  amphibia  is  caused  by  the  accumulation 
of  dark-colouring  matter  at  the  animal  pole  of  the  ovum.  In  consequence  of 
this,  the  animal  cells  of  the  ectoderm  are  darker  than  the  vegetable  cells  of  the 
entoderm.  We  find  the  reverse  of  this  in  the  case  of  most  animals,  the  proto- 
plasm of  the  entoderm  cells  being  usually  darker  and  coarser-grained. 


THE  GASTRULATION  OF  THE  VERTEBRATE 


lines  between  the  different  cells,  which  come  from  the  repeated 
segmentation  of  the  ovum,  look  like  deep  furrows  on  the 
surface,  and  hence  the  whole  process  has  been  given  the 
name  of  furcation.  In  reality,  however,  this  "segmentation," 
which  was  formerly  regarded  as  a  very  mysterious  process,  is 
nothing  but  the  familiar,  repeated  cell-segmentation.  Hence 
also  the  segmentation-cells  which  result  from  it  (the  segmen- 
tella  or  blastomeres)  are  real  cells. 

The  unequal  segmentation  which  we  observe  in  the  ovum 
of  the  amphibia  has  the  special  feature  of  beginning  at  the 
upper  and  darker  pole  (the  north  pole  of  the  terrestrial  globe 
in  our  illustration),  and  slowly  advances  towards  the   lower 
and    brighter    pole   (the   south    pole).     Also    the    upper  and 
darker  hemisphere   remains   in   this  position   throughout  the 
course  oi  the  segmentation,  and  its  cells  multiply  much  more 
briskly.     Hence  the  cells  of  the  lower  hemisphere  are  found 
to  be  larger  and  less  numerous.     The  cleavage  of  the  stem- 
cell    (Fig.  42  A)  begins  with    the    formation  of  a   complete 
meridian    furrow,    which    starts    from    the    north    pole   and 
reaches  to  the  south  ( B ).     An  hour  later  a  second  meridian 
furrow  arises  in  the  same  way,  and  this  cuts  the  first  at  a 
right  angle  (Fig.  42  C ).     The  ovum  is  thus  divided  into  four 
equal  parts.     Each  of  these  four  "  segmentation-cells  "  has  an 
upper  and  darker  and  a  lower  brighter  half.     A  few  hours 
later    a    third    furrow    appears,    vertically    to    the    first    two 
1  Fig.  42  D).     This  circular  furrow  is  usually,  but  improperly, 
called   the  "equatorial   furrow";    it  lies  to  the  north  of  the 
equator,  and  is  more  like  the  tropic  of  cancer.     The  globular 
germ  now  consists  of  eight  cells,  four  smaller  ones  above 
(northern)  and  four  larger  ones  below  (southern).     Next,  each 
of  the  four  upper  ones  divides  into  two  halves  by  a  meridian 
cleavage  beginning  from  the  north  pole,  so  that  we  now  have 
eight  above  and  four  below  (Fig.  42  E).     Later,  the  four  new 
meridian   divisions  extend    gradually  to  the  lower  cells,  and 
the  number  rises  from  twelve  to  sixteen (F).     Then  a  second 
circular  furrow  appears,  parallel  to  the  first,  and  nearer    to 
the  north  pole,  so  that  we  may  compare  it  to  the  north  polar 
circle.     In  this  way  we  get  twenty-four  segmentation-cells — 


178  THE  GASTRULATION  OF  THE  VERTEBRATE 

sixteen  upper,  smaller,  and  darker  ones,  and  eight  smaller 
and  brighter  ones  below  ( G).  Soon,  however,  the  latter 
also  sub-divide  into  sixteen,  a  third  or  "meridian  of  latitude  " 
appearing,  this  time  in  the  southern  hemisphere:  this  makes 
thirty-two  cells  altogether  ( ' H ).  Then  eight  new  meridian 
lines  are  formed  at  the  north  pole,  and  these  proceed  to 
divide,  first  the  darker  cells  above  and  afterwards  the  lighter 
southern  cells,  and  finally  reach  the  south  pole.  In  this  way 
we  get  in  succession  forty,  forty-eight,  fifty-six,  and  at  last 
sixty-four  cells  (I,  K).  In  the  meantime,  the  two  hemi- 
spheres differ  more  and  more  from  each  other.  Whereas  the 
sluggish  lower  hemisphere  long  remains  at  thirty-two  cells, 
the  lively  northern  hemisphere  briskly  sub-divides  twice,  pro- 
ducing first  sixty-four  and  then  128  cells  ( L,  M ).  Thus  we 
reach  a  stage  in  which  we  count  on  the  surface  of  the  ovum 
128  small  cells  in  the  upper  half  and  thirty-two  large  ones  in 
the  lower  half,  or  160  altogether.  The  dissimilarity  of  the 
two  halves  increases  :  while  the  northern  breaks  up  into  a 
great  number  of  small  cells,  the  southern  consists  of  a  much 
smaller  number  of  larger  cells.  Finally,  the  dark  cells  of  the 
upper  half  grow  almost  over  the  surface  of  the  ovum,  leaving 
only  a  small  circular  spot  at  the  south  pole,  where  the  large 
and  clear  cells  of  the  lower  half  are  visible.  This  white 
region  at  the  south  pole  corresponds,  as  we  shall  see  after- 
wards, to  the  primitive  mouth  of  the  gastrula.  The  whole 
mass  of  the  inner  and  larger  and  clearer  cells  (including  the 
white  polar  region)  belongs  to  the  entoderm  or  ventral  layer. 
The  outer  envelope  of  dark  smaller  cells  forms  the  ectoderm 
or  skin  layer. 

The  repeated  segmentation  which  can  thus  easily  be 
followed  on  the  surface  of  the  ovum  is  not  confined  to  the 
surface,  but  extends  to  the  whole  interior.  Thus,  the  cells 
divide  in  planes  which  correspond  pretty  closely  to  concen- 
tric planes  of  the  spherical  body  :  more  quickly  in  the  upper 
and  more  slowly  in  the  lower  half.  In  the  meantime,  a  large 
cavity,  full  of  fluid,  has  been  formed  within  the  globular  body 
— the  segmentation-cavity  or  embryonic-cavity  (blastocosl, 
Figs.    43-46  F,   and   also   J   in    the   transverse  sections   on 


THE  GASTRULATION  OF  THE  VERTEBRATE 


■79 


Plate  II.,  Figs.  8-1 1).  The  first  trace  of  this  cavity  is  found 
in  the  middle  of  the  upper  hemisphere,  where  the  first  three 
successive  planes  of  cleavage  cut  each  other  (Plate  II., 
Fig.  8  s ).     It  extends  considerably  by  progressive  cleavage, 

and      afterwards     assumes      an      almost     semi-circular    form 


1 


P        fl 
Fig.  44- 


Fig.  45. 


Fig.  4''. 


Figs.  43-46.— Four  vertical  sections  of  the  fertilised  ovum  of  the 
toad,  in  four  successive  stages  01  development.  The  Letters  have  the  same 
-  throughout: — F  segmentation-cavity.  D  covering  of  same  (  D  dorsal 
half  of  the  embryo,  P  ventral  half).  P yelk-stopper  (white  round  field  at  the 
lower  poll-).  Z  yelk-cells  of  the  entoderm  (Remak's  "  glandular  embryo"). 
X  primitive  gul  cavity  (progaster  or  Rusconian  alimentary  cavity).  The 
primitive  mouth  (prostoma)  is  closed  by  the  yelk-stopper,  P.  s  partition 
between  the  primitive  gut  cavity  (  X  ).  and  the  segmentation  cavity  (  F ).  k  k' 
il  the  large  circular  lip-border  of  the  primitive  mouth  (the  Rusconian 
anus).  The  line  of  dots  between  k  and  U  indicates  the  earlier  connection  of 
the  yelk-stopper  (P)  with  the  central  mass  of  tin-  yelk-cells  (Z).  In  V\^.  4<> 
tho  ovum  has  turned  90°,  so  that  the  back  of  the  embryo  is  uppermost  and 
the  ventral  side  down.     (  From  Strieker. ) 


(Fig.  4.;  F;  Plate  II.,  Figs.  9  s,  \os).  The  vaulted  roof  of 
this  hemispherical  segmentation-cavity  is  formed  by  the 
smaller  and  dark-coloured  cells  of  the  ectoderm  (Fig.  43  D ); 
Oil  the  other  hand,  its  level  floor  is  composed  of  the  larger 
and  lighter  cells  of  the  entoderm  (Fig.  43  z).     The  globular 


THE  GASTRULATION  OF  THE   VERTEBRATE 


frog-embryo  now  represents  a  modified  germinal  vesicle  or 
blastula,  with  hollow  animal  half  and  solid  vegetal  half. 

Now  a  second,  narrower  but  longer,  cavity  arises  by 
bending  from  the  lower  pole,  and  by  the  falling  away  from 
each  other  of  the  white  entoderm-cells  (Figs.  43-46  N).  This 
is  the  primitive  gut-cavity  or  the  gastric  cavity  of  the  gastrula, 
progaster  or  archenteron.  It  was  first  observed  in  the  ovum 
of  the  amphibia  by  Rusconi,  and  so  called  the  Rusconian 
alimentary  cavity.  In  vertical  section  (Fig.  44)  it  seems  to 
be  bent  in  the  form  of  a  sickle,  and  reaches  almost  from  the 
south  pole  to  the  north,  forcing  upwards  a  part  of  the  gut- 
cells  (between  the  segmentation-cavity  F  and  the  dorsal 
covering  D).  The  reason  of  the  peculiar  narrowness  of  the 
primitive  gut-cavity  here  is  that  it  is,  for  the  most  part,  full  of 
yelk-cells  of  the  entoderm.  These  also  stop  up  the  whole 
of  the  wide  opening  of  the  primitive  mouth,  and  form  what  is 
known  as  the  "yelk-stopper,"  which  is  seen  freely  at  the 
white  round  spot  at  the  south  pole  ( P).  Around  it  the 
ectoderm  is  much  thicker,  and  forms  the  border  of  the 
primitive  mouth  (the  properistoma ),  the  most  important  part 
of  the  embryo  (Fig.  46  k,  k).  Soon  the  primitive  gut-cavity 
stretches  further  and  further  at  the  expense  of  the  segmenta- 
tion-cavity ( F),  until  at  last  the  latter  disappears  altogether. 
The  two  cavities  are  only  separated  by  a  thin  partition  (Fig. 
45  s).  The  part  of  the  embryo  under  which  the  primitive 
gut-cavity  developes  is  the  later  dorsal-surface  ( D ).  The 
segmentation-cavity  lies  to  the  front  and  the  yelk-stopper  at 
the  hinder  part  of  the  body;  the  thick  hemispherical  mass  of 
the  yelk-cells  forms  the  ventral  wall  of  the  primitive  gut. 

With  the  formation  of  the  primitive  gut  our  frog-embryo 
has  reached  the  gastrula  stage  (Plate  II.,  Fig.  11).  But  it  is 
clear  that  this  cenogenetic  amphibian  gastrula  is  very  different 
from  the  real  palingenetic  gastrula  we  have  considered 
(Figs.  32-38).  In  the  latter,  the  be\\-ga.str\i\a.(archigastruhi )y 
the  body  has  only  one  axis.  The  primitive  gut-cavity  is 
empty  and  its  mouth  wide  open.  Both  the  ectoderm  and 
the  entoderm  consist  of  a  single  layer  of  cells.  They  lie 
close     together,     the     segmentation-cavity     having    wholly 


THE  GASTRVLATION  OF  THE  VERTEBRATE 


disappeared  in  the  process  of  folding.  It  is  quite  otherwise 
with  the  tufted  gastrula  (amphigastrula)  of  our  amphibia 
(Figs.  4,1-46  ;  Plate  II.,  Fig.  1  1).  In  this  case  the  segmenta- 
tion-cavity ( F)  remains  for  a  long  time  beside  the  primitive 
gut-cavitv  ( X ).  The  latter  is,  for  the  most  part,  filled  with 
yelk-cells,  and  the  primitive  mouth  almost  stopped  up  with 
them  (yelk-stopper,  P).  Both  entoderm  and  ectoderm  consist 
o(  several  layers  of  cells.  Finally  the  typical  form  of  the 
whole  gastrula  is  no  longer  uni-axial,  but  tri-axial ;  owing  to 
the  eccentric  development  of  the  primitive  gut-cavity,  the 
three  straight  axes  are  determined  which  characterise  the 
bilateral  body  of  the  higher  animals. 

In  the  growth  of  this 
tufted  gastrula  we  cannot 
sharply  mark  off  the  various 
stages  which  we  distinguish 
successively  in  the  bell- 
gastrula  as  mulberry-form 
and  vesicular  embryo.  The 
morula-stage  (Plate  II., 
Fig.  9)  is  no  more  clearly 
distinct  from  that  of  the 
blastula  (Fig.  10)  than  this 
is  from  the  gastrula  (Fig. 
11).  Nevertheless,  it  is 
not    difficult  to  reduce   the 


Fig.  47.  Embryonic  vesicle  of  the 
water-salamander  ( triton).  fh  seg- 
mentation-cavity, cte  yelk-cells,  ra  border- 
zone.     (  From  Hertwig.  | 


whole  cenogenetic  or  disturbed  development  of  this  amphi- 
gastrula to  the  true  palingenetic  formation  of  the  archi- 
gastrula  of  the  amphioxus. 

This  reduction  becomes  easier  if,  after  considering  the 
gastrulation  of  the  tailless  amphibia  (frogs  and  toads),  we 
glance  for  a  moment  at  that  of  the  tailed  amphibia,  the 
salamanders.  In  some  of  the  latter  that  have  only  recently 
been  carefully  studied,  and  that  are  phylogenetically  older, 
the  process  is  much  simpler  and  clearer  than  is  the  case 
with  the  former  and  longer  known.  Our  common  water- 
salamander  (triton  taeniatus)  is  a  particularly  j^ood  subject 
for  observation.     Its  nutritive  yelk  is  much  smaller  and  its 


182  THE  GASTRULATION  OF  THE  VERTEBRATE 

formative  yelk  less  troubled  with  black  pigment-cells  than  in 
the  case  of  the  frog  ;  and  its  gastrulation  has  better  retained 
the  original  palingenetic  character.  •  It  was  first  described  by- 
Scott  and  Osborn  (1879),  and  Oscar  Hertwig  especially  made 
a  careful  study  of  it  (1881),  and  rightly  pointed  out  its  great 
importance  in  helping  us  to  understand  the  vertebrate 
development. 

The  globular  embryonic  vesicle  of  iriton  (Fig.  47)  consists 
of  loosely-aggregated,  yelk-filled  entodermic  cells  or  yelk- 
cells  (dz)  in  the  lower  vegetal  half;  the  upper,  animal  half 
encloses  the  hemispherical  segmentation-cavity  (fh),  the 
curved  roof  of  which  is  formed  of  two  or  three  strata  of  small 
ectodermic  cells.  At  the  point  where  the  latter  pass  into  the 
former  (at  the  equator  of  the  globular 
vesicle)  we  have  the  border  zone  (rz). 
The  folding  which  leads  to  the  formation 
of  the  gastrula  takes  place  at  a  spot  in 
this  border  zone.  This  invagination- 
opening,  the  primitive  mouth  (Fig. 
48   it),    is    a    horizontal  transverse   fold 

„     _    .  with     a    dorsal    upper    lip    and    ventral 

Fig.  48.— Embryonic  .  rr  r 

vesicle  of  triton  (bias-     under   lip.       While    the    primitive    gut 

tula  J,    outer   view,    with       ,„.  ,  ,    .       ,  ,        .     . 

the  transverse  fold  of  (Fig.  49  ud)  is  being  bent  in,  a  part 
the  primitive  mouth  ( „ ).       f  th    secrmentation-cavity  (fh)  remains 

\Yrom  Hertivtg.)  &  j   \j     s 

at  first.  But  it  grows  smaller  (Fig.  49), 
and  finally  disappears.  In  the  complete  gastrula  (Fig.  50) 
the  external  germinal  layer  ( ak )  consists  of  a  single  laver 
of  high  cylindrical  cells.  The  internal  germinal  layer  ( ' ik ) 
is,  in  the  upper  and  dorsal  half,  also  composed  of  a  single 
stratum  of  cells  ;  these  form  the  covering  of  the  primitive  gut- 
cavity.  But  the  floor  of  the  latter,  or  the  lower  and  ventral 
half,  consists  of  several  layers  of  large  yelk-cells  (dz).  This 
part  of  the  entoderm,  which  is  also  known  as  the  yelk- 
embryo  ( lecithoblastus ),  is  much  smaller  in  the  water- 
salamander  than  in  the  frog.  Here,  again,  a  projection  of 
it  reaches  into  the  primitive  mouth  as  "yelk-stopper" 
(Fig.  50  p).  At  the  thick  borders  of  the  latter  begins  the 
formation  of  the  middle  germinal  layer  (ink ). 


THE  GASTRULATION  OF  THE   VERTEBRATE 


'83 


Fig.  4"  Sagittal  section  of  a  hooded- 
embryo  (depula)  of  triton  1  \  esicular  em- 
bryo at  the  commencement  of  gastrulation  1. 
til-  outer  germinal  layer,  ik  inner  terminal 
layer,  fh  segmentation-cavity,  ud  primi- 
tive gut.  11  primitive  mouth,  dl  and  vl 
dorsal  and  ventral  lips  of  the  mouth,  dz 
yelk-cells.     1  From  Hertmig. ) 


The  unequal  segmen- 
tation takes  place  in  some 
of  the  cyclostoma  and  in 

the  oldest  fishes  in  just 
the  same  way  as  in  most 
of  the  amphibia.  Among 
the  cyclostoma  ("  round- 
mouthed  ")  the  familiar 
lamprey s(petromyzontes  ) 
are  particularly  interest- 
ing. In  respect  of  organi- 
sation and  development 
they  are  half-way  between 
the  acrania  and  the 
lowest  real  fishes  ( scla- 
c/ui);  hence  I  divided 
the  group  of  the  cyclo- 
stoma in  1S66  from  the  real  fishes  with  which  they  were 
formerly  associated,  and  formed  of  them  a  special  class 
of  vertebrates.  The  ovum-segmentation  in  our  common 
river-lampreys  (petromyzon  fiwviatUis)  was  described  by 
Max  Schultze  in  1856,  and  afterwards  by  Scott  (1882)  and 
Goette  (1890). 

Unequal  total  segmentation  follows  the  same  lines  in  the 

oldest  fishes,  the  selachii 
and  ganoids,  which  are 
directly  connected  phy- 
logenetically  with  the 
cyclostoma.  The  primi- 
tive fishes  (selachii), 
which  we  must  regard  as 
the  ancestral  group  o( 
the  true  fishes,  were 
generally  considered 
until  a  short  time  ago 
to  be  discoblastic.  It 
was  not  until  the  begin- 
ning   of    the     twentieth 


l'n,.  50.  Sagittal  section  of  the  gas- 
trula  of  the  water-salamander  (triton). 
(From  Hertmig.)  Letters  as  in  Fig.  40; 
except — p  yelk-stopper,  mk  beginning  of 
the  middle  germinal  layer. 


1 84 


THE  GASTRULATIO.Y  OF  THE   VERTEBRATE 


century  that  Bashford  Dean  made  the  important  discovery 
in  Japan  that  one  of  the  oldest  living  fishes  of  the  shark 
type   (cestracion   japonicus )    has    the    same    total    unequal 


Fig-.  51.— Ovum-segmentation  in  the  lamprey  (petromyeonfluviatilis) 

in  four  successive  stages.  The  small  cells  of  the  upper  (animal)  hemisphere 
divide  much  more  quickly  than  the  cells  of  the  lower  (vegetal)  hemisphere. 

segmentation  as  the  amphiblastic  plated  fishes  (ganoides).z 
This  is  particularly  interesting  in  connection  with  our 
subject,    because     the     few    remaining     survivors    of     this 


Fig.  52.  —  Gastrulation  of  the 
lamprey  (petromyson  fiuviatilis).  A 
blastula,  with  wide  embryonic  cavity 
(blastocoel,  bl),  g  incipient  invagination. 
B  depula,  with  advanced  invagination, 
from  the  primitive  mouth  ( g).  C  gas- 
trula.  with  complete  primitive  gut  :  the 
embryonic  cavity  has  almost  disappeared 
in  consequence  of  invagination. 


'  Bashford   Dean,   Holoblastic  Cleavage  in  the  Egg  of  a  Shark,    cestracion 
japonicus  Macleay.     Annotationes  soologicae  japonenses,  vol.  iv.,  Tokio,  1901. 


THE  GASTRULATION  OF  THE  VERTEBRATE  185 

division,  which  was  so  numerous  in  paleozoic  times,  exhibit 
three  different  types  oi'  gastrulation.  The  oldest  and  most 
conservative  forms  of  the  modern  ganoids  are  the  scaley 
sturgeons  (sfurtones),  plated  fishes  of  great  phyletic  impor- 
tance, ilie  eggs  of  which  are  eaten  as  caviare  ;  their  cleavage 
is  not  essentially  different  from  that  of  the  petromyzontes  and 
amphibia.  On  the  other  hand,  the  most  modern  of  the 
plated  fishes,  the  beautifully  scaled  bony  pike  of  the  North 
American  rivers  ( lepidosteus ),  approaches  the  osseous  fishes, 
and  is  discoblastic  like  them.  A  third  genus  (amia)  is 
midway  between  the  sturgeons  and  the  latter. 

The  group  of  the  lung-fishes  (dipneusta  or  dipnoi )  is 
closely  connected  with  the  older  ganoids.  In  respect  of  their 
whole  organisation  they  are  midway  between  the  gill- 
breathing  fishes  and  the  lung-breathing  amphibia  ;  they 
share  with  the  former  the  shape  of  the  body  and  limbs,  and 
with  the  latter  the  form  of  the  heart  and  lungs.  Of  the  older 
dipnoi  (paladipneusta)  we  have  now  only  one  specimen,  the 
remarkable  ceratodus  of  East  Australia  ;  its  amphiblastic 
gastrulation  has  been  recently  explained  by  Richard  Semon 
(cf.  Chapter  XXI).  That  of  the  two  modern  dipneusta,  of 
which  protopterus  is  found  in  Africa  and  lepidosiren  in 
America,  is  not  materially  different.      (Cf.  Fig.  53.) 

All  these  amphiblastic  vertebrates,  petromyzon  and 
cestracion,  accipenser  and  ceratodus,  and  also  the  salamanders 
and  batrachia,  belong  to  the  old,  conservative  groups  of  our 
stem.  Their  unequal  ovum-segmentation  and  gastrulation 
have  many  peculiarities  in  detail,  but  can  always  be  reduced 
with  comparative  ease  to  the  original  cleavage  and  gastrula- 
tion of  the  lowest  vertebrate,  the  amphioxus  ;  and  this  is  little 
removed,  as  we  have  seen,  from  the  very  simple  archigastrula 
of  the  sagitta  and  monoxenia  (see  p.  152,  Figs.  31-38).  All 
these  and  many  other  classes  of  animals  generally  agree  in 
the  circumstance  that  in  segmentation  their  ovum  divides 
into  a  large  number  of  cells  by  repeated  cleavage.  All  such 
ova  have  been  called,  after  Remak,  "whole-cleaving"  f/10/0- 
blasta),  because  their  division  into  cells  is  complete  or  total 
(Plate  II.). 


i86  THE  GASTRULATION  OF  THE   VERTEBRATE 

In  a  great  many  other  classes  of  animals  this  is  not  the 
case,  as  we  find  (in  the  vertebrate  stem)  among  the  birds, 
reptiles,  and  most  of  the  fishes  ;  among  the  insects  and  most 
of  the  spiders  and  crabs  (of  the  articulates)  ;  and  the  cephalo- 
pods  (of  the  molluscs).  In  all  these  animals  the  mature 
ovum,  and  the  stem-cell  that  arises  from   it  in  fertilisation, 


fh  gh 


Fig.  53.— Gastrulation  of  ceratodus  (from 

Semon).  A  and  C  stage  with  four  cells,  B  and 
D  with  sixteen  cells.  A  and  B  are  seen  from 
above,  C  and  D  sideways.  £  stage  with  thirty- 
two  cells  ;  .Fblastula;  G  gastrula in  longitudinal 
section,  fh  segmentation  cavity,  gh  primitive 
gut  or  gastric  cavity. 


consists  of  two  different  and  separate  parts,  which  we  have 
called  formative  yelk  and  nutritive  yelk.  The  formative  yelk 
(vitclhis  formativus  or  morpholecithus )  alone  consists  of 
living  protoplasm,  and  is  the  active,  evolutionary,  and 
nucleated  part  of  the  ovum  ;  this  alone  divides  in  segmentation, 


THE  GASTRULATION  OF  THE  VERTEBRATE  187 


and  produces  the  numerous  cells  which  make  up  the  embryo. 
On  the  other  hand,  the  nutritive  yelk  (vitellus  nutritivus 
or  tropholecithus )  is  merely  a  passive  part  of  the  contents 
of  the  ovum,  a  subordinate  element  which  contains  nutritive 
material  or  deutoplasm  (albumin,  fat,  etc.).  and  SO  represents 
in  a  sense  the  provision-store  of  the  developing  embryo. 
The  latter  takes  a  quantity  of  food  out  oi  this  store,  and 
finally  consumes  it  all.  Hence  the  nutritive  yelk  is  of  great 
indirect  importance  in  embryonic  development,  though  it  has 
no  direct  share  in  it.  It  either  does  not  divide  at  all,  or  only 
later  on,  and  does  not  generally  consist  of  cells.  It  is  some- 
times large  and  sometimes  small,  but  generally  many  times 
larger  than  the  formative  yelk  ;  and  hence  it  is  that  it  was 
formerly  thought  the  more  important  of  the  two.  As  the 
respective  significance  of  these  two  parts  of  the  ovum  is  often 
wrongly  described,  it  must  be  borne  in  mind  that  the  nutritive 
yelk  is  only  a  secondary  addition  to  the  primary  cell  ;  it  is  an 
inner  enclosure,  not  an  external  appendage.  All  ova  that 
have  this  independent  nutritive  yelk  are  called,  after  Remak, 
"partially-cleaving"  ( meroblasta).  Their  segmentation  is 
incomplete  or  partial  (Plate  III.). 

There  are  many  difficulties  in  the  way  of  understanding 
this  partial  segmentation  and  the  gastrula  that  arises  from  it. 
We  have  only  recently  succeeded,  by  means  of  comparative 
research,  in  overcoming  these  difficulties,  and  reducing  this 
cenogenetic  form  of  gastrulation  to  the  original  palingenetic 
type.  This  is  comparatively  easy  in  the  small  meroblastic 
ova  which  contain  little  nutritive  yelk — for  instance,  in  the 
pelagic  ova  of  a  bony  fish,  the  development  of  which  I 
observed  in  i.S;5at  Ajaccio  in  Corsica  (Plate  III.,  Figs.  1.S-24). 
I  found  them  joined  together  in  lumps  of  jelly,  floating  on 
the  surface  of  the  sea  ;  and  as  the  little  ovula  were  com- 
pletely transparent,  I  could  easily  follow  the  development  of 
the  germ  step  by  step.  These  ovula  are  glossy  and  colourless 
globules  of  little  more  than  half  a  millimetre  in  diameter 
(0.64-0.66  mm).  Inside  a  structureless,  thin,  but  firm  mem- 
brane ((/oolemma.  Fig.  54  c  )  we  find  a  large,  quite  clear,  and 
transparent  globule  of  albumin  (d).     At  both   poles   o\   its 


THE  GASTRULATION  OF  THE  VERTEBRATE 


axis  this  globule  has  a  pit-like  depression.  In  the  pit  at  the 
upper,  animal  pole  (which  is  turned  downwards  in  the  floating 
ovum)  there  is  a  bi-convex  lens  composed  of  protoplasm,  and 
this  encloses  the  nucleus  (k);  this  is  the  formative  yelk  of 
the  stem-cell,  or  the  germinal  disk  (b).  From  the  neighbour- 
hood of  this  lense-shaped  nutritive  yelk  a  very  thin  proto- 
plasmic skin  spreads  around,  and  this  protects  the  nutritive 
yelk,  the  "  border-layer."  At  the  opposite  or  vegetal  pole  of 
the  ovum,  in  the  lower  pit,  there  is  a  clear  simple  globule  of 
fat  (f).  The  small  fat-globule  and  the  large  albumin-globule 
together  form  the  nutritive  yelk. 
Only  the  formative  yelk  under- 
goes cleavage,  the  nutritive  yelk 
not  dividing  at  all  at  first. 

The  segmentation  of  the  lens- 
shaped  formative  yelk  (b)  pro- 
ceeds quite  independently  of  the 
nutritive  yelk,  and  in  perfect 
geometrical  order  (cf.  Plate  III., 
Figs.  18-24;  or|ly  tne  formative 
yelk  with  the  nearest  part  of  the 
nutritive  yelk  (n)  is  given  in 
section  [through  a  meridian 
plane]  in  this  illustration,  the 
greater  part  of  the  latter  and  the 
The  stem-cell  (Fig.  18)  first 
divides  into  two  equal  segmentation-cells  (Fig.  19).  From 
these  we  get  by  repeated  sub-division  first  four,  then 
eight,  then  sixteen  cells  (Fig.  20).  By  continued  cleavage 
we  then  get  thirty-two  cells,  sixty-four,  and  so  on.  All 
these  segmentation-cells  are  at  first  of  the  same  size  and 
character.  Closely  joined  together,  they  form  a  lens-shaped 
mass  (Plate  III.,  Fig.  21),  something  like  the  globular 
mulberry  -  embryo  of  the  primordial  cleavage  (morula, 
Plate  II.,  Fig.  5).  But  afterwards  the  border  cells  of  the 
lens  separate  from  the  rest,  and  travel  into  the  yelk  and  the 
border-layer;  they  form  the  "  embryonic  border"  (periblast, 
Fig-  55   C,  p).     From  this  lens-shaped   mulberry-form  there 


Fig.  54.— Ovum  of  a  pelagic 

bony  fish,  b  protoplasm  of  the 
stem-cell.  /■  nucleus  of  same. 
d  clear  globule  of  albumin,  the 
nutritive  yelk,  f  fat-g-lobule  of 
same,  c  outer  membrane  of  the 
ovum,  or  ovolemma. 


ovolemma   being  left   out). 


THE  GASTRULATION  OF  THE   VERTEBRATE 


|S) 


then  developes  a  vesicular  embryo  (blastula),  the  cells  of  the 
periblast  making  their  way  centripetally  underneath  the  lens 
(Plate  III.,  Fig.  22).  The  regular  bi-convex  lens  is  converted 
into  a  disk  like  a  watch-glass  with  thick  borders.  This 
convex  cell-disk  lies  on  the  upper  and  less  curved  polar 
surface  of  the  nutritive  yelk  like  the  watch-glass  011  the 
watch.  As  fluid  gathers  in  the  space  between  the  blastoderm 
and  the  periblast,  a  round  low  cavity  is  formed  (Fig.  22  s). 
This  is  the  segmentation-cavity,  corresponding  to  the  central 
segmentation-cavity  of  the  palingenetic  blastula  (Plate  II., 
Fig.  4).  The  slightly  curved  floor  of  the  lower  segmentation 
cavity  is  formed  by  the  periblast  and  nutritive  yelk  (11  J,  and 


Fig.  5.v—  Ovum-segmentation  of  a  bony  fish.  (Cf.  Plate  III.,  Figs. 
1S-24.)  .1  first  cleavage  of  the  stem-cell  (cytula).  J!  division  of  same  into 
tour  segmentation-cells  (only  two  visible).  C  the  germinal  disk  divides  into 
the  blastoderm  (b)  and  the  periblast  CpJ-  <i  nutritive  yolk,  /'fat-globule. 
c  ovolemma.  -  space  between  the  ovolemma  and  the  ovum,  tilled  with  a  clear 
fluid. 

the  greatlv  curved  roof  of  it  by  the  blastula-cells.  Our  fish- 
embryo  is  now  really  a  vesicle  with  eccentric  cavity,  like  the 
blastula  of  the  frog  (Plate  II.,  Fig.  10)  and  the  salamander 
(Fig.  47).  But,  whereas  in  the  case  of  these  amphibia  the 
larger  vegetal  half  of  the  blastula  is  formed  of  the  big  yelk- 
cells,  in  our  bony  fish  it  is  taken  up  with  the  periblast  and 
the  structureless,  undivided  nutritive  yelk. 

Then  follows  the  important  process  of  invagination,  which 
leads  to  the  formation  of  the  gastrula.  As  a  result  oi  a 
further  enlargement  and  displacement  of  the  blastula-cells,  the 
thick  borders  of  the  cell-disk,  which  lie  on  the  nutritive  yelk, 
grow  centripetally  inwards  towards  the  middle  of  the  segmen- 
tation-cavity (Fig.  23  s).     The  invagination,  which  may  also 


THE  GASTRCLATIOX  OF  THE  VERTEBRATE 


be  conceived  as  a  turning-up  of  the  border  of  the  blastoderm, 
begins  at  a  spot  that  corresponds  to  the  edge  of  the  primitive 
mouth  or  the  later  anus.  The  inner,  hollowed-out  layer, 
consisting  of  one  simple  stratum  of  cells,  is  the  entoderm  ;  it 
is  immediately  attached  from  the  under  side  to  the  upper, 
several-layered  part  of  the  embryonic  membrane,  the  ecto- 
derm. In  this  process  the  segmentation  cavity  disappears. 
The  space  underneath  the  entoderm  corresponds  to  the 
primitive  gut-cavity,  and  is  filled  with  the  decreasing  food- 
yelk  (n ).  Thus  the  formation  of  the  gastrula  of  our  fish  is 
complete. 

In  contrast  to  the  two  chief 
forms  of  gastrula  we  considered 
previously,  we  give  the  name  of 
discoid  gastrula  ( discogastrula, 
Fig.  56)  to  this  third  principal 
type.  As  a  fact,  the  mass  of  cells 
that  compose  it  represent  a  cir- 
cular, concave-convex  thin  disk. 
This  disk  is  attached  by  its  inner, 
hollow  side  to  the  curved  surface 
of  the  nutritive  yelk  ( 11 ).  Its 
outer  surface  is  rounded  con- 
vexly  like  a  shield.  If  we  make 
a  horizontal  section  through  the 
middle  of  the  gastrula  (in  a 
meridian  plane  cf  the  globular 
composed  of  several  strata  (four 
in  the  present  case)  of  cells  (Plate  III.,  Fig.  24).  Directly 
over  the  food-yelk  lies  a  single  stratum  of  larger  cells 
(Fig.  24  /),  which  have  a  soft,  thick,  coarse-grained  proto- 
plasm, and  colour  dark-red  with  carmine.  These  form  the 
gut-layer  or  entoderm,  and  arise  from  the  growth  of  the 
borders  of  the  disk  (folded  germinal  layer).  The  three 
outer  strata  that  lie  on  it  form  the  skin-layer  or  ectoderm 
(Fig.  24  e).  They  consist  of  smaller  cells,  that  take  very 
little  colour  in  carmine  ;  their  protoplasm  is  firmer,  clearer, 
and  finer-grained.     At   the  thickened  edge  of  the   gastrua, 


Fig.  56.— Discoid  gastrula 
(discogastrula)  of  a  bony  fish. 
e  ectoderm,  i  entoderm.  -,<>  bor- 
der-swelling or  primitive  mouth. 
n  albuminous  globule  of  the  nutri- 
tive velk.  f  fat-globule  of  same. 
c  external  membrane  (ovolemma). 
d  partition  between  entoderm  and 
ectoderm  I  earlier  the  segmenta- 
tion cavity). 

ovum),   we    find  that  it  is 


THE  GASTRULATION  OF  THE  VERTEBRATE  191 

the  primitive-mouth  edge  (border-swelling  or  properistoma), 

the  entoderm  and  ectoderm  pass  into  each  other  without 
definite  limit  (Fig.  5010). 

Ot  late  years  this  discoid  gastrulation  of  the  bony  fishes 
has  been  very  carefully  described  by  Kupffer,  Van  Bambeke, 
Whitman,  Wilson,  Kopsch,  H.  E.  Ziegler,  and  others.  In 
most  of  the  teleostei  it  is  more  complicated  and  changed 
cenogenetically,  because  the  food-yelk  is  very  large  and  forms 
an  extensive  globular  body,  an  emulsion  of  albumin  and  fat- 
particles.  During  the  growth  of  the  lens-shaped  germinal 
disk  a  part  of  the  nucleus  at  the  border  of  it  travels  into  the 
yelk,  and  forms  what  is  called  a  periblast,  which  surrounds 
the  blastoderm  like  a  ring.  The  incompletely  divided  yelk- 
cells  of  the  periblast  that  are  thus  formed  are  also  called 
'•  yclk-syncytium  ";  they  are  used  upas  food  by  the  embryo 
with  the  rest  of  the  yelk,  and  have  no  part  in  the  building-up 
of  the  body.  The  same  applies  to  the  covering-layer,  a 
.simple  thin  stratum  of  flat  epithelial  cells,  which,  in  many 
fishes,  forms  the  uppermost  layer  of  the  blastoderm,  and  at 
its  border  connects  with  the  contiguous  part  of  the  periblast, 
the  germinal  wall.1 

Very  similar  to  the  discoid  gastrulation  of  the  osseous 
fishes  is  that  of  the  myxinoida,  the  remarkable  cyclostoma 
that  live  parasitically  in  the  body-cavity  of  fishes,  and  are 
distinguished  by  several  notable  peculiarities  from  their 
nearest  relatives,  the  lampreys  (petromyzon).  While  the 
amphiblastic  ova  of  the  latter  are  small  and  develop  like 
those  of  the  amphibia,  the  cucumber-shaped  ova  of  the 
myxinoida  are  several  centimetres  long,  and  form  a  discoid 
gastrula.  Up  to  the  present  it  has  only  been  observed  in  one 
species  (bdellostoma  Stouti),  by  Dean  and  Doflein  (1898). 

It  is  clear  that  the  important  features  which  distinguish 
the  discoid  gastrula  from  the  other  chief  forms  we  have  con- 
sidered are  determined  by  the  large  food-yelk.  This  takes 
no  direct   part  in   the    building  o(   the  germinal    layers,   and 

1  Cf.  Kingsley  and  Conn,  Embryology  of  the  Teleosts  (1883);  A.  Agassiz 
anil  C.  O.  Whitman,  The  Development  0/ Osseous  Fishes  (1885);  M'Intosh, 
Development  and  Life-histories  of  Fishes  ( 1890). 


192  THE  GASTRVLATIOX  OF  THE   VERTEBRATE 

completely  fills  the  primitive  gut-cavity  of  the  gastrula,  even 
protruding  at  the  mouth-opening.  If  we  imagine  the  original 
bell-gastrula  (Figs.  32-38)  trying  to  swallow  a  ball  of  food 
which  is  much  bigger  than  itself,  it  would  spread  out  round 
it  in  discoid  shape  in  the  attempt,  just  as  we  find  to  be  the 
case  here  (Fig.  56).  Hence  we  may  derive  the  discoid  gas- 
trula  from  the  original  bell-gastrula,  through  the  inter- 
mediate stage  of  the  tufted  gastrula.  It  has  arisen  phylo- 
genetically  by  the  accumulation  of  a  store  of  food-stuff  at 
the  vegetal  pole,  a  "nutritive  yelk"  being  thus  formed  in 
contrast  to  the  "formative  yelk."  Nevertheless,  the  gastrula 
is  formed  here,  as  in  the  previous  cases,  by  the  folding  or 
invagination  of  the  blastula.  We  can,  therefore,  reduce  this 
cenogenetic  form  of  the  discoid  segmentation  ( gastrulatio 
discoidalis J  to  the  palingenetic  form  of  the  primitive  cleavage. 
This  reduction  is  tolerably  easy  and  confident  in  the  case 
of  the  small  ovum  of  our  pelagic  bony  fish,  but  it  becomes 
difficult  and  uncertain  in  the  case  of  the  large  ova  that  we 
find  in  the  majority  of  the  other  fishes  and  in  all  the  reptiles 
and  birds.  In  these  cases  the  food-yelk  is,  in  the  first  place, 
comparatively  colossal,  the  formative  yelk  being  almost 
invisible  beside  it ;  and,  in  the  second  place,  the  food-yelk 
contains  a  quantity  of  different  elements,  which  are  known  as 
"yelk-granules,  yelk-globules,  yelk-plates,  yelk-flakes,  yelk- 
vesicles,"  and  so  on.  Frequently  these  definite  elements  in 
the  yelk  have  been  described  as  real  cells,  and  it  has  been 
wrongly  stated  that  a  portion  of  the  embryonic  body  is  built 
up  from  these  cells.1  This  is  by  no  means  the  case.  In 
every  case,  however  large  it  is — and  even  when  cell-nuclei 
travel  into  it  during  the  cleavage  of  the  blastoderm-border, 
and  form  a  periblast — the  nutritive  yelk  remains  a  dead 
accumulation  of  food,  which  is  taken  into  the  gut  during 
embryonic  development  and  consumed  by  the  embryo.  The 
latter  developes  solely  from  the  living  formative  yelk  of  the 

'  The  coll-like  matter  fruit  we  find  in  the  undivided  food-yelk  of  birds, 
reptiles,  and  fishes  is  anything  but  true  cells,  as  His  and  others  affirm.  The 
true  cells  which  we  find  in  the  food-yelk  of  these  meroblastic  ova  after  cleavage 
are  migrated  segmentation-cells  (merocyta,  Fig.  447.) 


THE  GASTRULATION  OF  TIIF.   VERTEBRATE  193 

sti.-ni-ji.-ll.  This  is  equally  true  o(  the  ova  of  our  small  bony 
fishes  and  of  the  colossal  ova  of  the  primitive  fishes,  reptiles, 
and  birds. 

The  gastrulation  of  the  primitive  fishes  or  selachii  (sharks 
and  rays)  has  been  carefully  studied  of  late  years  by  Riickert, 
Rabl,  and  H.  E.  Ziegler  in  particular,  and  is  very  important 
in  the  sense  that  this  group  is  the  oldest  among  living  fishes, 
and  their  gastrulation  can  be  derived  directly  from  that  of  the 
cyclostoma  by  the  accumulation  of  a  large  quantity  of  food- 
yelk.  The  oldest  sharks  (cestracion)  still  have  the  unequal 
segmentation  inherited  from  the  cyclostoma.  But  while  in 
this  case,  as   in  the  case  of  the  amphibia,  the  small    ovum 


Fig.  57.—  Longitudinal  section  through  the  blastula  of  a  shark 
(pristiuris).  (From  Riickert.)  (Looked  at  from  the  left  ;  to  the  right  is  the  hinder 
end,  //.  to  the  lefl  the  fore  end,  V.)  />'  segmentation-cavity,  he  cells  of  the 
germinal  membrane,  dk  yelk-nuclei. 

completely  divides  into  cells  in  segmentation,  this  is  no 
longer  so  in  the  great  majority  of  the  selachii  (or  elasmo- 
branchii).  In  these  the  contractility  of  the  active  protoplasm 
no  longer  suffices  to  break  up  the  huge  mass  of  the  passive 
deutoplasm  completely  into  cells  ;  this  is  only  possible  in  the 
upper  or  dorsal  part,  but  not  in  the  lower  or  ventral  section. 
Hence  we  find  in  the  primitive  fishes  a  blastula  with  a  small 
eccentric  segmentation-cavity  (Fig.  57  b ),  the  wall  of  which 
varies  greatly  in  composition.  Only  the  roof  (or  upper  wall) 
of  it  consists  of  real  blastodermic  cells,  and  forms  the 
germinal  disk  (kz);  the  floor  or  lower  wall  is  formed  of 
undivided  yelk-stuff,  in  which  the  presence  of  "elementary 
organisms"  is  only  indicated  by  scattered  yelk-granules  (dk). 


THE  GASTRULATION  OF  THE  VERTEBRATE 


The  circular  border  of  the  germinal  disk  or  the  thin 
"transition  zone,"  which  connects  the  roof  and  floor  of  the 
segmentation-cavity,  corresponds  to  the  border-zone  at  the 
equator  of  the  amphibian  ovum.  In  the  middle  of  its  hinder 
border  we  have  the  beginning  of  the  invagination  of  the 
primitive  gut  (Fig.  58  ud);  it  extends  gradually  from  this 
spot  (which  corresponds  to  the  Rusconian  anus  of  the 
amphibia)  forward  and  around,  so  that  the  primitive  mouth 
becomes  first  crescent-shaped  and  then  circular,  and,  as  it 
opens  wider,  surrounds  the  ball  of  the  larger  food-yelk  ( disco- 
gastrula  eurvstoma ).  Not  only  the  obviously  divided 
cylindrical  cells  of  the  roof  (the  blastocytes),  but  also  the 
contiguous  parts  of  the  yelk  that  contain  the  yelk-nuclei  (  dk  j 

v 


dk 

Fig.  58.— Longitudinal  section  of  the  blastula  of  a  shark  (pristiurus) 

at  the  beginning'  of  gastrulation.  (From  Ruder/. )  (Seen  from  the  left. )  Kfore 
end,  H  hind  end,  B  segmentation-cavity  or  blastocoel,  ud  first  trace  of  the 
primitive  gut,  dk  yelk-nuclei,  fd  fine-grained  yelk,  gd  coarse-grained  yolk. 


or  the  nuclei  of  the  still  undivided  merocytes,  take  part  in  the 
invagination.  As  these  gradually  divide  and  become  inde- 
pendent round  entodermic  cells,  they  form  the  ventral  wall  of 
the  primitive  gut ;  its  dorsal  wall  is  made  up  of  the 
cylindrical  cells  which  are  formed,  in  a  continuous  simple 
layer,  at  the  inner  side  of  the  roof  of  the  segmentation-cavity 
during  the  advancing  invagination.  The  cavity  is  thus 
pressed  in  on  this  side  also,  and  displaced  by  the  cavity  of  the 
primitive  gut  (ud).  But  only  the  back  wall  of  this  wide- 
mouthed  discoid  gastrula  continues  for  some  time  to  consist 
of  two  distinct  strata  of  cells  (the  primary  germinal  layers),  its 
ventral  wall  being  composed  of  the  yelk-stuff.  As  .this 
gradually    disappears,    the    wide    primitive    mouth    becomes 


I  HE  C.  I S  TS  ULA  TION  OF  THE  l  '/•;/,•  TF.IiR.  I  TE  195 

smaller.     In    this   discoid    gastrula   the    ventral    lip   of    the 
primitive  mouth  is  in  front,  the  dorsal  lip  behind. 

Essentially  different  from  this  wide-mouthed  discogastrula 
of  most  of  the  selachii  is  the  epigastrula  (of  Rabl),  the 
narrow -mouthed  discoid  gastrula  of  the  amniotes,  the  reptiles, 
birds,  and  monotremes ;  between  the  two — as  a  phylogenetic 
intermediate  stage — we  have  the  holoblastic  amphigastrula  of 
the  amphibia.  The  latter  has  developed  from  the  amphi- 
gastrula of  the  ganoids  and  dipneusts,  whereas  the  discoid 
amniote  gastrula  has,  in  turn,  evolved  from  the  amphibian 
gastrula  by  the  addition  of  food-yelk.  This  phylogenetic 
change  of  gastrulation  is  still  found  in  the  remarkable  ophidia 
( gymnophionayc(Bcilia,  orperomelaj,  serpent-like  amphibia  that 
live  in  moist  soil  in  the  tropics,  and  in  many  respects  repre- 
sent the  transition  from  the  gill-breathing  amphibia  to  the 
lung-breathing  reptiles.  Their  embryonic  development  has 
been  explained  by  the  fine  studies  of  the  brothers  Sarasin  of 
ichthyophis  glutmosa  at  Ceylon  (1887),  and  those  of  August 
Brauer  of  the  hypogeopliis  rostrata  in  the  Seychelles  (1897). 
It  is  only  by  the  historical  and  comparative  study  of  these 
that  we  can  understand  the  difficult  and  obscure  gastrulation 
of  the  amniotes. 

The  bird's  egg  is  particularly  important  for  our  purpose, 
because  most  of  the  chief  studies  of  the  development  of  the 
vertebrates  are  based  on  observations  of  the  hen's  egg  during 
hatching.  The  mammal  ovum  is  much  more  difficult  to 
obtain  and  study,  and  for  this  practical  and  obvious  reason 
very  rarely  thoroughly  investigated.  But  we  can  get  hens' 
eggs  in  any  quantity  at  any  time,  and,  by  means  of  artificial 
incubation,  follow  the  development  of  the  embryo  step  by 
step.  The  bird's  egg  differs  considerably  from  the  tiny 
mammal  ovum  in  size,  a  large  quantity  of  food-yelk  accumu- 
ating  within  the  original  yelk  or  the  protoplasm  of  the  ovum. 
This  is  the  yellow  ball  which  we  commonly  call  the  yelk  of 
the  egg.  In  order  to  understand  the  bird's  egg  aright — for 
it  is  very  often  quite  wrongly  explained — we  must  examine  it 
in  its  original  condition,  and  follow  it  from  the  very  beginning 
of  its  development  in   the  bird's  ovary.     We  then  see  that 


ig6 


THE  GASTRULATIOX  OF  THE   VERTEBRATE 


the  original  ovum  is  a  quite  small,  naked,  and  simple  cell 
with  a  nucleus,  not  differing  in  either  size  or  shape  from  the 
original  ovum  of  themammalsand  otheranimals  (cf.  Fig.  13-fi). 
As  in  the  case  of  all  the  craniota,  the  original  or  primitive 
ovum  ( protovum  )  is  covered  with  a  continuous  layer  of  small 
cells,  like  an  epithelium.  This  epithelial  membrane  is  the 
follicle,  from  which  the  ovum  afterwards  issues.  Immediately 
underneath  it  the  structureless  yelk-membrane  is  secreted 
from  the  yelk. 

The  small  primitive  ovum  of  the 
bird  begins  very  early  to  take  up 
into  itself  a  quantity  of  food-stuff 
through  the  yelk-membrane,  and  work 
it  up  into  the  "yellow  yelk."  In  this 
way  the  ovum  enters  on  its  second 
stage  (the  metovum  j,  which  is  many 
times  larger  than  the  first,  but  still 
only  a  single  enlarged  cell.  Through 
the  accumulation  of  the  store  of 
yellow  yelk  within  the  ball  of  proto- 
plasm the  nucleus  it  contains  (the 
germinal  vesicle)  is  forced  to  the 
surface  of  the  ball.  Here  it  is  sur- 
rounded by  a  small  quantity  of  proto- 
plasm, and  with  this  forms  the  lens- 
shaped  formative  yelk  (Fig.  59  b). 
This  is  seen  on  the  yellow  yelk- 
ball,  at  a  certain  point  of  the  surface, 
as  a  small  round  white  spot — the  "  scar  "  f  cicatriculaj.  From 
this  scar  a  thread-like  column  of  white  nutritive  yelk  (dj, 
which  contains  no  yellow  yelk-granules,  and  is  softer  than  the 
yellow  food-yelk,  proceeds  radially  to  the  middle  of  the 
yellow  yelk-ball,  and  forms  there  a  small  central  globule  of 
white  yelk  (Fig.  59  d).  The  whole  of  this  white  yelk  is  not 
sharply  separated  from  the  yellow  yelk,  which  shows  a 
slight  trace  of  concentric  layers  in  the  hard-boiled  egg  (Fig. 
59  c).  We  also  find  in  the  hen's  egg,  when  we  break  the 
shell  and  take  out  the  yelk,  a  round  small  white  disk  at  its 


Fig.  59.— A  ripe  ovum 
from  the  ovary  of  a  hen 

(in  section).  The  yellow 
food-yelk  is  composed  of 
concentric  layers  (c),  and 
surrounded  by  a  thin  yelk- 
membranef'rt^.  The  nucleus 
or  germinal  vesicle  forms, 
with  the  protoplasm  of  the 
ovum,  the  formative  yelk 
(b)  or  the  "scar."  From 
this  the  white  yelk  (here 
dark)  goes  into  the  yelk- 
cavity  (d ).  But  the  two 
kinds  of  yelk  are  not  sharply 
distinct. 


THE  GASTRULATION  OF  THE  VERTEBRATE  197 

surface  which  corresponds  to  the  scar.  But  this  small  white 
"germinal  disk"  is  now  further  developed,  and  is  really  the 
gastrula  of  the  chick.  The  body  of  the  chick  is  formed  from 
it  alone.  The  whole  white  and  yellow  yelk-mass  is  without 
any  significance  for  the  formation  of  the  embryo,  it  being 
merely   used  as  food   by  the  developing  chick.      The  clear, 


Fig.  "i.  Diagram  ol  discoid  segmentation  in  the  bird's  ovum 
(magnified  about  ton  times).  Only  the  formative  yolk  (the  scar)  is  shown  in 
these  -i\  figures  (  A-F ),  because  cleavage  only  takes  place  in  this.  The  much 
larger  food-yelk,  which  does  not  share  in  the  cleavage,  is  left  out  and  merely 
indicated  by  the  dark  ring-  without.  .1  By  the  first  division  the  ovinia  splits 
into  two  cells.  B  These  two  first  segmentation-cells  divide  by  a  second 
cleavage  (vertical  to  the  firsii  into  four  cells.  C  From  these  tour  cells  sixteen 
arc  formed,  two  other  radial  divisions  taking  place  between  the  first  two 
transverse  divisions,  and  the  inner  ends  of  these  eight-rayed  segments  being 
cut  off  by  a  central  ring-cleavage.  />  A  stage  with  sixteen  peripheral  and 
some  four  concentric  radial  clefts.  E  A  stage  with  sixty-four  peripheral  and 
six  circular  clefts.  F  By  continuous  repetition  of  radial  and  circular  divisions 
tin'  whole  soar  breaks  into  a  heap  of  small  cells,  and  now  forms  the  lens-shaped 
mulberry-type  (morula).  The  division  of  the  nuclei  .always  precedes  the 
formation  o\'  clefts. 


glarous  mass  of  albumin  that  surrounds  the  yellow  yelk  of 
the  bird's  egg,  and  also  the  hard  calcareous  shell,  are  only 
formed  within  the  oviduct  round  the  impregnated  ovum. 

When  the  fertilisation  of  the  bird's  ovum  has  taken  place 
within  the  mother's  body,  we  find  in  the  lens-shaped  stem-cell 
the  progress  of  flat,  discoid  segmentation  {gastrula  discoidalis, 
Fig.  60).      First  two  equal  segmentation-cells  (A  )  are  formed 


198  THE  GASTRULATION  OF  THE  VERTEBRATE 

from  the  cytula.  These  divide  into  four  ( B J,  then  into  eight, 
sixteen  (C),  thirty-two,  sixty-four,  and  so  on.  The  cleavage 
of  the  cells  is  always  preceded  by  a  division  of  their  nuclei. 
The  cleavage  surfaces  between  the  segmentation-cells  appear 
at  the  free  surface  of  the  scar  as  clefts.  The  first  two  divisions 
are  vertical  to  each  other,  in  the  form  of  a  cross  ( B J.  Then 
there  are  two  more  divisions,  which  cut  the  former  at  an 
angle  of  forty-five  degrees.  The  scar,  which  thus  becomes 
the  germinal  disk,  now  has  the  appearance  of  an  eight-rayed 
star.  A  circular  cleavage  next  taking  place  round  the  middle, 
the  eight  triangular  cells  divide  into  sixteen,  of  which  eight  are 
in  the  middle  and  eight  distributed  around  (  C).  Afterwards 
circular  clefts  and  radial  clefts,  directed  towards  the  centre, 
alternate  more  or  less  irregularly  ( D,  E).  In  most  of  the 
amniotes  the  formation  of  concentric  and  radial  clefts  is 
irregular  from  the  very  first  ;  and  so  also  in  the  hen's  egg. 
But  the  final  outcome  of  the  cleavage-process  is  once  more 
the  formation  of  a  large  number  of  small  cells  of  a  similar 
nature.  As  in  the  case  of  the  fish-ovum,  these  segmentation- 
cells  form  a  round,  lens-shaped  disk,  which  corresponds  to 
the  mulberry-embryo,  and  is  embedded  in  a  small  depression 
of  the  white  yelk.  Between  the  lens-shaped  disk  of  the 
morula-cells  and  the  underlying  white  yelk  a  small  cavity  is 
now  formed  by  the  accumulation  of  fluid,  as  in  the  fishes. 
Thus  we  get  the  peculiar  and  not  easily  recognisable  blastula 
of  the  bird  (Fig.  61).  The  small  segmentation-cavity  (fh)  of 
this  notably  cenogenetic  blastula  is  very  fiat  and  much  com- 
pressed. The  upper  or  dorsal  wall  (dw)  is  formed  of  a  single 
layer  of  clear,  distinctly  separated  epithelial  cells  ;  this  corre- 
sponds to  the  upper  or  animal  hemisphere  of  the  triton- 
blastula  (Fig.  47).  The  lower  or  ventral  wall  of  the  flat 
dividing  space  (vw)  is  made  up  of  larger  and  darker 
segmentation-cells,  which  are  in  part  not  yet  separated,  and 
pass  directly  into  the  substance  of  the  underlying  white  yelk 
(wd);  it  corresponds  to  the  lower  or  vegetal  hemisphere  of 
the  blastula  of  the  water-salamander  (Fig.  47  da  J.  The 
nuclei  of  the  yelk-cells,  which  are  in  this  case  especially 
numerous  at  the   edge    of  the    lens-shaped    blastula,    travel 


THE  GASTRULATION  OF  THE  VERTEBRATE 


(as   merocytes)   into   the  white   yelk,    increase    by  cleavage, 

and  contribute  oven  to  the  further  growth  of    the    germinal 
disk  by  furnishing  it  with  food-stuff. 

The  invagination  or  the  typical  folding  of  the  bird- 
blastula  takes  place  in  this  case  also  at  the  hinder  (aboral) 
pole  of  the  subsequent  chief  axis,  in  the  middle  of  the  hind 


Fig.  '.i. 

hi  vl  i.  dk 


Fig.  63. 


Fu;.  62. 


Fig.  61.— Vertical  section  of  the  blastula  of  a  hen  (discoblastula). 
fh  segmentation-cavity,  </:.•  dorsal  wall  of  same,  vw  ventral  wall,  passing 
directly  into  the  white  yelk  (vdj.     (  From  /)«;•«/.) 

Fig.  62.    The  germinal  disk  of  the  hen's  ovum  at  the  beginning  of 

gastrulation  :  .  I  before  incubation,  />'  in  the  first  hour  of  incubation.  (From 
Koller.)  ks  germinal  disk.  V  it-,  lore  and  //  its  hind  border;  cs  embryonic 
shield  ;  s  sickle-groove  ;  sk  sickle  knob  ;  d  yelk. 

Fig.  63.  Longitudinal  section  of  the  germinal  disk  of  a  siskin 
(discogastrula).  (From  DuvaL)  ud  primitive  gut,  vl,  M  fore  and  hind  lips  oi 
the  primitive  mouth  (or  sickle-edge) ;  ok  outer  germinal  layer,  ik  inner  germinal 
layer,  ilk  yelk-nuclei,  wd  white  yelk. 

border  of  the  round  germinal  disk  (Fig.  62  s).  At  this  spot 
we  have  the  most  brisk  cleavage  of  the  cells  ;  hence  the  cells 
are  more  numerous  and  smaller  here  than  in  the  fore-half  of 
the  germinal  disk.  The  border-swelling  or  thick  edge  oi'  the 
disk  is  less  clear  but  whiteF  behind,  and  is  more  sharply 
separated  from  contiguous  parts.  In  the  middle  of  its  hind 
border   there    is   a    white,   crescent-shaped    groove — Roller's 


200  THE  GASTRULATIOX  OF  THE  VERTEBRATE 

sickle-groove  (Fig.  62  s) ;  a  small  projecting  process  in  the 
centre  of  it  is  called  the  sickle-knob  ( sk).  This  important 
cleft  is  the  primitive  mouth,  which  was  described  for  a  long 
time  as  the  "primitive  groove."  If  we  make  a  vertical 
section  through  this  part  (in  the  middle  or  sagittal  plane),  we 
see  that  a  flat  and  broad  cleft  stretches  under  the  germinal 
disk  forwards  from  the  primitive  mouth  ;  this  is  the  primitive 
gut  (Fig.  63  ltd).  Its  roof  or  dorsal  wall  is  formed  by  the 
folded  upper  part  of  the  blastula,  the  segmentation-cavity 
of  which  is  now  only  visible  as  an  insignificant  channel, 
bordered  above  by  the  simple  cell-layer  of  the  outer  germinal 
layer  (ak),  and  below  by  the  inner  germinal  layer  with  its 
several  strata  (ik).  The  floor  or  the  ventral  wall  of  the  flat 
primitive  gut  is  formed  by  the  white  yelk  fwd),  in  which  a 


Fig.    64.— Longitudinal  section  of  the   diseoid   gastrula  of  the 

nightingale.  (From  Duval.)  ml  primitive  gut,  -■/,  hi  fore  and  hind  lips  of 
the  primitive  mouth  ;  <//•,  ik  outer  and  inner  germinal  layers  ;  VI  fore-border  of 
the  discogastrula. 

number  of  yelk-nuclei  (dk )  are  distributed.  There  is  a  brisk 
multiplication  of  these  merocytes  at  the  edge  of  the  germinal 
disk,  especially  in  the  neighbourhood  of  the  sickle-shaped 
primitive  mouth. 

We  learn  from  sections  through  later  stages  of  this  discoid 
bird-gastrula  that  the  primitive  gut-cavity,  extending  forward 
from  the  primitive  mouth  as  a  flat  pouch,  undermines  the 
whole  region  of  the  round  flat  lens -shaped  blastula 
(Fig.  64  ud).  At  the  same  time,  the  segmentation-cavity 
gradually  disappears  altogether,  the  folded  inner  germinal 
layer  (ik)  placing  itself  from  underneath  on  the  overlying 
outer  germinal  layer  (ak).  The  typical  process  of  invagina- 
tion, though  greatly  disguised,  can  thus  be  clearly  seen  in 
this  case,  as  Goette  and  Rauber,  and  more  recently  Duval 
(Fig.  64),  have  shown. 


THE  GASTRULATION  OF  THE  VERTEBRATE 


The  older  embryologists  (Pander,  Baer,  Remak),  and,  in 
recent  times  especially,  His,  Kolliker,  and  others,  said 
that  the  two  primary  germinal  layers  of  the  hen's  ovum — the 
oldest  and  most  frequent  subject  of  observation  ! — arose  by 
horizontal  cleavage  of  a  simple  germinal  disk.  In  opposition 
to  this  accepted  view.  I  affirmed  in  my  Gastrcea  Theory  (1873) 
that  the  discoid  bird-gastrula,  like  that  of  all  other  verte- 
brates, is  formed  by  folding  (or  invagination),  and  that  this 
typical  process  is  merely  altered  in  a  peculiar  way  and 
disguised  by  the  immense  formation  of  spherical  food-yelk 
and  the  flat  spreading  of  the  discoid  blastula  at  one  part  of 
its  surface.  I  endeavoured  to  establish  this  view  by  the 
monophyletic  derivation  of  the  vertebrates,  and  especially  by 
proving  that  the  birds  descend  from  the  reptiles,  and  these 
from  the  amphibia.  If  this  is  correct,  the  discoid  gastrula  of 
the  amniotes  must  have  been  formed  by  the  folding-in  of  a 
hollow  blastula,  as  has  been  shown  by  Remak  and  Rusconi 
of  the  discoid  gastrula  of  the  amphibia,  their  direct  ancestors. 
The  accurate  and  extremely  careful  observations  of  the 
authors  I  have  mentioned  (Goette,  Rauber,  and  Duval)  have 
decisively  proved  this  recently  for  the  birds  ;  and  the  same  has 
been  done  for  the  reptiles  by  the  fine  studies  of  Kupffer, 
Beneke,  Wenkebach,  and  others.  In  the  shield-shaped 
germinal  disk  of  the  lizard  (Fig.  65),  the  crocodile,  the 
tortoise,  and  other  reptiles,  we  find  in  the  middle  of  the  hind 
border  (at  the  same  spot  as  the  sickle  groove  in  the  bird)  a 
transverse  furrow  ( 11  J,  which  leads  into  a  fiat,  pouch-like, 
blind  sac,  the  primitive  gut.  The  fore  (dorsal)  and  hind 
(ventral)  lips  of  the  transverse  furrow  correspond  exactly  to 
the  lips  of  the  primitive  mouth  (or  sickle-groove)  in  the  birds. 

The  gastrulation  of  the  mammals  must  be  derived  from 
this  special  embryonic  development  of  the  sauropsida  (reptiles 
and  birds).  This  latest  and  most  advanced  class  of  the 
vertebrates  has,  as  we  shall  see  afterwards,  evolved  at  a 
comparatively  recent  date  from  an  older  group  of  reptiles,  the 
tocosauria;  and  all  these  amniotes  must  have  come  originally 
from  a  common  older  stem-form,  the  protamniota  or  pro- 
reptilia.     Hence   the   distinctive   embryonic   process    of    the 


202  THE  GASTRULATION  OF  THE   VERTEBRATE 

mammal  must  have  arisen  by  cenogenetic  modifications  from 
the  older  form  of  gastrulation  of  the  sauropsida.  Until  we 
admit  this  thesis  we  cannot  understand  phylogenetically  the 
formation  of  the  germinal  layers  in  the  mammal,  and  there- 
fore in  man. 

I  first  advanced  this  fundamental  principle  in  my  essay 
On  the  Gastrulation  of  Mammals  (1877),  and  sought  to  show 
in  this  way  that  I  assumed  a  phylogenetic  degeneration  of 
the  food-yelk  and  the  yelk-sac  on  the  way  from  the  pro- 
reptiles   to   the   mammals.       "The    cenogenetic    process    of 


Fig.  65.— Germinal  disk  Of  the  lizard  (lacerta  agilisj.  (From  Kupffer.} 
u  primitive  mouth,  s  sickle,  es  embryonic  shield,  hf  and  <//'  light  and  dark 
Ererminative  area. 


adaptation,"  I  said,  "which  has  occasioned  the  atrophy  of  the 
rudimentary  yelk-sac  of  the  mammal,  is  perfectly  clear.  It  is 
the  adaptation  to  the  lengthy  stay  in  the  womb  of  the  vivi- 
parous mammal,  whose  ancestors  were  certainly  oviparous. 
As  the  great  store  of  food-yelk,  which  the  oviparous  ancestors 
gave  to  the  egg,  became  superfluous  in  their  descendants 
owing  to  the  long  carrying  in  the  womb,  and  the  maternal 
blood  in  the  wall  of  the  uterus  made  itself  the  chief  source 
of  nourishment,  the  now  useless  yelk-sac  was  bound  to 
atrophy  by  embryonic  adaptation." 


the  gastrulation  of  the  vertebrate 


\lv  opinion  met  with  little  approval  at  the  time;  it  was 
vehemently  attacked  by  Kolliker,  Hensen,  and  His  in 
particular.  However,  it  has  been  gradually  accepted,  and 
has  recently  been  firmly  established  by  a  large  number  of 
excellent  studies  of  mammal  gastrulation,  especially  by 
Edward  Van  Beneden's  studies  o{  the  hare  and  bat, 
Selenka's  on  the  marsupials  and  rodents,  Heape's  and 
Lieberkiihn's  on  the  mole,  Kupffer  and  Keibel's  on  the 
rodents,  Bonnet's  on  the  ruminants,  etc.  From  the  general 
Comparative  point  of  view,  Carl  Rabl  in  his  theory  of  the 
mesoderm,  Oscar  Hertwig  in  the  latest  edition  o(  his  Manual 
(1902),  and  Hubrecht  in  his  Studies  in  Mammalian  Embry- 
ology (1891),  have  supported  the  opinion,  and  sought  to 
derive  the  peculiarly  modified  gastrulation  of  the  mammal 
from  that  of  the  reptile. 

In  the  meantime  (1884)  the  studies  of  Wilhelm  Haacke 
and  Caldwell  provided  a  proof  of  the  long-suspected  and  very 
interesting  fact,  that  the  lowest  mammals  and  the  monotremes 
lay  eggs,  like  the  birds  and  reptiles,  and  are  not  viviparous 
like  the  other  mammals.  Although  the  gastrulation  of  the 
monotremes  was  not  reallv  known  until  studied  by  Richard 
Semon  in  1894,  there  could  be  little  doubt,  in  view  of  the 
great  size  of  their  food-yelk,  that  their  ovum-segmentation 
was  discoid,  and  led  to  the  formation  of  a  sickle-mouthed 
discogastrula,  as  in  the  case  of  the  reptiles  and  birds.  Hence 
I  had,  in  1875  (in  my  essay  on  The  Gastrula  and  Ovum- 
segmentation  of  Animals),  counted  the  monotremes  among 
the  discoblastic  vertebrates.  This  hypothesis  was  established 
as  a  fact  nineteen  years  afterwards  by  the  careful  observations 
o\  Semon;  he  gave  in  the  second  volume  of  his  great  work, 
Zoological  Journeys  in  Australia  (1894),  the  first  description 
and  correct  explanation  of  the  discoid  gastrulation  of  the 
monotremes.  The  fertilised  ova  of  the  two  living  monotremes 
(echidna  and  orni/horhynchus )  are  balls  of  4-5  mm.  diameter, 
enclosed  in  a  stiff  shell;  but  they  grow  considerably  during 
development,  so  that  when  laid  the  tgg  is  three  times  as 
large  (15-16  mm.).  The  structure  of  the  plentiful  yelk,  and 
especially  the  relation  of  the  yellow  and  the  white  yelk,  are 


THE  GASTRULATIOX  OF  THE   VERTEBRATE 


just  the  same  as  in  the  sauropsida.  As  with  these,  partial 
cleavage  takes  place  at  a  spot  on  the  surface  at  which  the 
small  formative  yelk  and  the  nucleus  it  encloses  are  found. 
First  is  formed  a  lens-shaped  circular  germinal  disc  (blasto- 
discus).  This  is  made  up  of  several  strata  of  cells,  but  it 
spreads  over  the  yelk-ball,  and  thus  becomes  a  one-layered 
blastula.  If  we  then  imagine  the  yelk  it  contains  to  be 
dissolved  and  replaced  by  a  clear  liquid,  we  have  the 
characteristic  blastula  (vesicula  blastodennica)  of  the  higher 
mammals.     In  these  the  gastrulation  proceeds  in  two  phases, 

as  Semon  rightly  ob- 
serves :  firstly,  formation 
of  the  cenogenetic  ento- 
derm by  delamination 
at  the  centre  and  further 
growth  at  the  periphery  ; 
secondly,  invagination. 
In  the  monotremes  more 
primitive  conditions 
have  been  retained  better 
than  in  the  reptiles  and 
birds.  In  these  saurop- 
sida before  the  com- 
mencement of  the 
gastrula  -  folding,  we 
have,  at  least  at  the 
periphery,  a  two-layered  embryo  forming  from  the  cleavage. 
But  in  the  monotremes  the  formation  of  the  cenogenetic 
entoderm  does  not  precede  the  invagination  ;  hence  in  this 
case  the  construction  of  the  germinal  layers  is  less  modified 
than  in  the  other  amniota. 

The  marsupials  come  next,  as  a  second  sub-class,  to  the 
oviparous  monotremes,  the  oldest  of  the  mammals.  But  as 
in  their  case  the  food-yelk  is  already  atrophied,  and  the  little 
ovum  developes  within  the  mother's  body,  the  partial  cleavage 
has  been  reconverted  into  total.  One  section  of  the  mar- 
supials still  show  points  of  agreement  with  the  monotremes, 
while   another   section    of  them,   according   to   the   splendid 


Fig.  66.— Ovum  of  the  opossum  (didel- 

phys)  divided  into  four.  (From  Selenka.) 
b  the  four  blastomeres,  r  directive  body,  c 
unnucleated  coagulated  matter,  p  albumin- 
membrane. 


THE  CASTKC/.ATJOX  OF  THE   VERTEBRATE 


investigations  of  Selenka,  form   a  connecting-link   between 
these  and  the  placentals. 

The  fertilised  ovum  of  the  opossum  (didelphys)  divides, 
according  to  Selenka,  first  into  two,  then  four,  then  eight 
equal  cells  ;  hence  the  segmentation  is  at  first  equal  or  homo- 
geneous. But  in  the  course  of  the  cleavage  a  larger  cell, 
distinguished  by  its  less  clear  plasm  and  its  containing  more 
yelk-granules  (the  mother-cell  of  the  entoderm.  Fig.  67  en), 
separates  from  the  other  blastomeres  ;  the  latter  multiply  more 
rapidly  than  the  former.  As,  further,  a  quantity  o(  fluid 
gathers  in  the  morula,  we  get  a  spherical  blastula,  the  wall  of 
which  is  of  varying  thickness,  like  that  of  the  amphioxus 
(Fig.  40  E)  and  the  a 
amphibia  (Fig.  47).  The 
upper  or  animal  hemi- 
sphere is  formed  01  a 
large  number  of  small 
cells  ;  the  lower  or  vegetal 
hemisphere  of  a  small 
number  of  large  cells. 
One  of  the  latter,  distin- 
guished by  its  size  (Fig. 
67  en),  lies  at  the  vegetal 
pole  of  the  blastula-axis, 
at  the  point  where  the 
primitive  mouth  after- 
wards appears.     This   is 

the  mother-cell  of  the  entoderm  ;  it  now  begins  to  multiply 
by  cleavage,  and  the  daughter-cells  (Fig.  68  i)  spread  out 
from  this  spot  over  the  inner  surface  of  the  blastula,  though 
at  first  only  over  the  vegetal  hemisphere.  The  less  clear 
entodermic  cells  (i)  are  distinguished  at  first  by  their  rounder 
shape  and  darker  nuclei  from  the  higher,  clearer,  and  longer 
ectodermic  cells  fej;  afterwards  both  are  greatly  flattened, 
the  inner  blastodermic  cells  more  than  the  outer. 

The  unnucleated  yelk-balls  and  curd  (Fig.  68  d)  that  we 
find  in  the  fluid  of  the  blastula  in  these  marsupials  are  very 
remarkable;    they   are    the    relics    of    the    phylogenetically 


Fig.  07.  Blastula  of  the  opossum 
(didelphys),  (From  Selenka.)  a  animal  pole 
of  the  blastula,  v  vegetal  pole,  en  mother- 
cell  of  the  entoderm,  ex  ectodermic  cells, 
s  spermia,  ib  unnucleated  yelk-balls  (remain- 
der of  the  food-yelk),  f  albumin-membrane. 


jo6  THE  GASTRULATION  OF  THE  VERTEBRATE 

atrophied  food-yelk,  which  was  developed  in  their  ancestors, 
the  monotremes,  and  in  the  reptiles. 

In  the  further  course  of  the  gastrulation  of  the  opossum 
the  oval  shape  of  the  gastrula  (Fig.  69)  gradually  changes 
into  globular,  a  larger  quantity  of  fluid  accumulating  in  the 
vesicle.  At  the  same  time  the  entoderm  spreads  further  and 
further  over  the  inner  surface  of  the  ectoderm  ( e).  A 
globular  vesicle  is  formed,  the  wall  of  which  consists  of 
two    thin    simple    strata    of   cells ;    the    cells    of    the    outer 


^;. 


r-w 


Fig.  68.  Fig.  69. 

Fig.    68.  — Blastula   Of   the   opossum  (didelphys)  at   the   beginning   of 

gastrulation.  (From  Selenka.)  e  ectoderm,  i  entoderm,  a  animal  pole,  u 
primitive  mouth  at  the  vegetal  pole, /"segmentation-cavity,  d unnucleated  yelk- 
balls  (relics  of  the  reduced  food-yelk),  c  nucleated  curd  (without  yelk-granules). 

Fig.  69.— Oval  gastrula  Of  the  OpOSSUm  (didelphys),  about  eight  hours 
old.     (From  Selenka)  (external  view). 

germinal  layer  are  rounder  and  those  or  the  inner  layer 
flatter.  In  the  region  of  the  primitive  mouth  (p)  the  cells 
are  less  flattened,  and  multiply  briskly.  From  this  point — 
from  the  hind  (ventral)  lip  of  the  primitive  mouth,  which 
extends  in  a  central  cleft,  the  primitive  groove — the  construc- 
tion of  the  mesoderm  proceeds. 

Gastrulation  is  still  more  modified  and  curtailed  cenoge- 
netically  in  the  placentals  than  in  the  marsupials.  It  was 
first  accurately  known  to  us  by  the  distinguished  investiga- 
tions of   Edward  Van   Beneden   in    1875,   the  first  object  of 


THE  GASTRULATION  OF  THE  VERTEBRATE 


study  being  the  ovum  of  the  hare.  But  as  man  also  belongs 
to  this  sub-class,  and  as  his  as  yet  unstudied  gastrulation 
Cannot  be  materially  different  from  that  o(  the  other  placentals, 
it  merits  the  closest  attention.  We  have,  in  the  first  place,  the 
peculiar  feature  that  the  two  first  segmentation-cells  that 
proceed  from  the  cleavage  of  the  fertilised  ovum  (Fig,  71) 
are  o\  different  sizes  and  natures  ;  the  difference  is  sometimes 
greater,  sometimes  less  (Fig.  72).  One  of  these  first  daughter- 
cells  oi  the  cytula — or  the  first  two  blastomeres — is  a  little 
larger,  clearer,  and  more  transparent  than  the  other.  Further, 
the  smaller  cell  takes  a  colour 
in  carmine,  osmium,  etc., 
more  Strongly  than  the 
larger.  By  repeated  cleavage 
of  it  a  morula  is  formed,  and 
from  this  a  blastula,  which 
changes  in  a  very  charac- 
teristic way  into  the  greatly 
modified  gastrula.  When 
the  number  of  the  segmenta- 
tion-cells in  the  mammal 
embryo  has  reached  ninety- 
six  (in  the  hare, about  seventy 

hours     after     impregnation)         ..  .  •     ,  ... 

Fir.    -,v  —Longitudinal    section 


the   fcetus  assumes  a   form  through  the  oval  gastrula  of  the 

...  ,              ,  .               ,  opossum   (Fig.   69).      (From   Selenka.) 

very    like  the   archigastrula  p  primitive  mouth,  e  ectoderm,  i  ento- 

/  I.-:  ,      -.  .  rc       piafo      n  derm,  1/  yelk  remains  in  the  primitive 

v     '&■     Oi  "■      ^i«*ie      ii.,  gut-cavity  (u). 

Fig.    17,    in    section).     The 

spherical  embryo  consists  oi  a  central  mass  of  thirty-two  soft, 
round  cells  with  dark  nuclei,  which  are  flattened  into  poly- 
gonal shape  by  mutual  pressure,  and  colour  dark-brown  with 
OSmic  acid  (Fig.  75  i).  This  dark  central  group  of  cells  is 
surrounded  by  a  lighter  spherical  membrane,  consisting  o( 
sixty-four  cube-shaped,  small,  and  fine-grained  cells  which  lie 
close  together  in  a  single  stratum,  and  only  colour  slightly  in 
osrnic  acid  ( Fig.  75  e).  The  authors  who  regard  this  embryonic 
form  as  the  primary  gastrula  of  the  placental  conceive  the 
outer  layer  as  the  ectoderm   and   the   inner  as   the  entoderm. 


2oS  THE  GASTRULATIOX  OF  THE  VERTEBRATE 

The  ectodermic  membrane  is  only  interrupted  at  one  spot,  one, 
two,  or  three  of  the  entodermic  cells  being  loose  there.  These 
form  the  yelk-stopper,  and  fill  up  the  mouth  of  the  gastrula  (a). 
The   central  primitive  gut-cavity  (d)    is  full  of  entodermic 


Fig.  7 


Fig.  71.— Stem-cell  or  eytula  of  the  mammal  ovum  (from  the  hare). 
k  stem-nucleus,  n  nuclear  corpuscle,  p  protoplasm  of  the  stem-cell,  z  modified 
zona  pellucida,  h  outer  albuminous  membrane,  5  dead  sperm-cells. 

Fig.  72. —Incipient  cleavage  of  the  mammal  ovum  (from  the  hare). 

The  stem-cell   has   divided    into   two   unequal   cells,   one   lighter  (e)   and  one 
darker  (i ).  z  zona  pellucida,  /;  outer  albuminous  membrane,  s  dead  sperm-cells. 


Fig.  73.  Fig.  74. 

Fig.  73.— The  first  four  segmentation-cells  of  the  mammal  ovum 
(from  the  hare),     e  The  two  larger  (and  lighter)  cells,  i  the  two  smaller  (and 

darker)  cells,  z  zona  pellucida,  h  outer  albuminous  membrane. 

Fig.   74.— Mammal  ovum  with  eight  segmentation-cells  (from  the 

hare),     e  four  larger  and   lighter  blastomeres,  ;  four  smaller  and  darker  cells, 
c  zona  pellucida,  h  outer  albuminous  membrane. 


THE  GASTRULATION  OF  THE  VERTEBRATE 


cells  (Plate  II.,  Fig.  17).  The  uni-axial  type  of  the  mammal 
gastrula  is  accentuated  in  this  way.  However,  opinions  still 
differ  considerably  as  to  the  real  nature  of  this  "  provisional 
gastrula"  o(  the  placental  and  its  relation  to  the  blastula  into 
which  it  is  converted. 

As  the  gastrulation  proceeds  a  large  spherical  blastula  is 
formed  from  this  peculiar  solid  amphigastrula  of  the  placental, 
as  we  saw  in  the  case  of  the  marsupial.  The  accumulation 
of  fluid  in  the  solid  gastrula  (Fig.  76  A)  leads  to  the  forma- 
tion of  an  eccentric  cavity,  the  group  of  the  darker  entodermic 
cells  '  Iiv )  remaining  directly 
attached  at  one  spot  with  the 
globular  enveloping  stratum 
of  the  lighter  eetodermic  cells 
(ep  ).  This  spot  corresponds 
to  the  original  primitive 
mouth  (prostoma  or  blasto- 
porus).  From  this  important 
spot  the  inner  germinal  laser 
spreads  all  round  on  the 
inner  surface  of  the  outer 
layer,  the  cell-stratum  of 
which  forms  the  wall  of  the 
hollow  sphere  ;  the  exten- 
sion proceeds  from  the 
vegetal  towards  the  animal 
pole. 

The  cenogenetic  gastru- 
lation of  the  placental  has  been  greatly  modified  by  secondary 
adaptation  in  the  various  groups  of  this  most  advanced  and 
youngest  sub-class  of  the  mammals.  Thus,  for  instance,  we 
find  in  many  of  the  rodents  (guinea-pigs,  mice,  etc. )  apparently 
a  temporary  inversion  of  the  two  germinal  layers.  This  is  due 
to  a  folding  of  the  blastodermic  wall  by  what  is  called  the 
"girder,"  a  plug-shaped  growth  of  Rauber's  "roof-layer." 
1 1  is  a  thin  layer  of  flat  epithelial  cells,  that  is  freed  from  the 
surface  of  the  blastoderm  in  some  of  the  rodents  ;  it  has  no 
more  significance  in  connection  with  the  general  course  o( 


l'n-  75.  Gastrula  of  the  placental 
mammal  (epigastrula  from  the  hare), 
longitudinal  section  through  the  axis, 
r  eetodermic  cells  (sixty-four,  lighter 
and  smaller),  i  entodermic  cells  (thirty- 
two,  darker  and  larger),  d  central  ento- 
dermic cell,  filling  the  primitive  gut- 
cavity,  o  peripheral  entodermic  cell, 
stopping  up  the  opening  of  the  primitive 
mouth  (yelk-stopper  in  the  Rusconian 
anus). 


210  THE  GASTRULATION  OF  THE   VERTEBRATE 

placental  gastrulation  than  the  conspicuous  departure  from 
the  usual  globular  shape  in  the  blastula  of  some  of  the 
ungulates.  In  some  pigs  and  ruminants  it  grows  into  a 
thread-like,  long  and  thin  tube. 

Thus  the  gastrulation  of  the  placentals,  which  diverges 
most  from  that  of  the  amphioxus,  the  primitive  form,  is 
reduced  to  the  original  type,  the  invagination  of  a  modified 
blastula.  Its  chief  peculiarity  is  that  the  folded  part  of  the 
blastoderm  does  not  form  a  completely  closed  (only  open  at 
the  primitive  mouth)  blind  sac,  as  is  usual  ;  but  this  blind 
sac  has  a  wide  opening  at  the  ventral  curve  (opposite  to  the 
dorsal  mouth)  ;  and  through  this  opening  the  primitive  gut 


Fig.  76. — Gastrula  Of  the  hare.  Jasa  solid,  spherical  cluster  of  cells, 
B  changing  into  the  embryonic  vesicle,  bp  primitive  mouth,  cfi  ectoderm,  hy 
entoderm. 

communicates  from  the  first  with  the  embryonic  cavity  of  the 
blastula.  The  folded  crest-shaped  entoderm  grows  with  a 
free  circular  border  on  the  inner  surface  of  the  entoderm 
towards  the  vegetal  pole  ;  when  it  has  reached  this,  and  the 
inner  surface  of  the  blastula  is  completely  grown  over,  the 
primitive  gut  is  closed.  This  remarkable  direct  transition  of 
the  primitive  gut-cavity  into  the  segmentation-cavity  is 
explained  simply  by  the  assumption  that  in  most  of  the 
mammals  the  yelk-mass,  which  is  still  possessed  by  the 
oldest  forms  of  the  class  (the  monotremes)  and  their  ancestors 
(the  reptiles),  is  atrophied.  This  proves  the  essential  unity 
of  gastrulation  in  all  the  vertebrates,  in  spite  of  the  striking 
differences  in  the  various  classes. 


THE  GASTRULATION  OF  THE  VERTEBRATE  zn 

In  order  to  complete  our  consideration  of  the  important 
processes  of  segmentation  and  gastrulation,  we  will,  in 
conclusion,  cast  a  brief  glance  at  the  fourth  chief  type — 
superficial  segmentation  (Plate  III.,  Figs.  25-30).  In  the 
vertebrates  this  form  is  not  found  at  all.  But  it  plays  the 
chief  part  in  the  large  stem  of  the  articulates — the  insects, 
spiders,  myriapods,  and  crabs.  The  distinctive  form  of 
gastrula  that  comes  of  it  is  the  "  vesicular  gastrula "  (peri- 
gastrula,  Plate  III.,  Fig.  29). 

In  the  ova  which  undergo  this  superficial  cleavage  the 
formative  yelk  is  sharply  divided  from  the  nutritive  yelk,  as 
in  the  preceding  cases  of  the  ova  of  birds,  reptiles,  fishes, 
etc.;  the  formative  yelk  alone  undergoes  cleavage.  But 
while  in  the  telolecithal  ova  with  discoid  gastrulation  the 
formative  velk  is  not  in  the  centre,  but  at  one  pole  of  the 
uni-axial  ovum,  and  the  food-yelk  gathered  at  the  other  pole, 
in  the  ova  with  superficial  cleavage  we  find  the  formative 
yelk  spread  over  the  whole  surface  of  the  ovum;  it  encloses 
spherically  the  food-yelk,  which  is  accumulated  in  the  middle 
of  the  centrolecithal  ova.  As  the  segmentation  only  affects 
the  former  and  not  the  latter,  it  is  bound  to  be  entirely 
"superficial";  the  store  of  food  in  the  middle  is  quite 
untouched  by  it.  As  a  rule,  it  proceeds  in  regular  geometrical 
progression  (Plate  III.,  Figs.  25-30,  illustrates  some  stages  of 
it  in  vertical  section  through  the  ellipsoid  ova  of  a  crab, 
pencils).  The  stem-nucleus,  or  first  segmentation-nucleus, 
which  is  situated  originally  in  the  centre  of  the  stem-cell, 
divides  into  two,  then  four,  eight,  and  finally  sixteen  nuclei. 
These  travel  centrifugally  out  of  the  central  food-yelk,  and 
distribute  themselves  at  equal  distances  in  the  superficial 
formative  yelk  (Plate  III.,  Fig.  26).  Mere  they  multiply 
continuously  by  cleavage  (Fig.  27).  Finally  the  whole  of 
the  formative  yelk  divides  into  a  number  of  small  and 
homogeneous  cells,  which  lie  close  together  in  a  single 
stratum  on  the  entire  surface  of  the  ovum,  and  form  a  super- 
ficial blastoderm  (Fig.  286).  This  blastoderm  is  a  simple, 
completely  closed  vesicle,  the  internal  cavity  of  which  is 
entirely  full  of  food-yelk.     This   real   blastula  (Fig.  28)  only 


212  THE  GASTRULATION  OF  THE   VERTEBRATE 

differs  from  that  of  the  archiblastic  ova  (Plate  II.,  Fig.  4)  in 
its  chemical  composition.  In  the  latter  the  content  is  water 
or  a  watery  jelly  ;  in  the  former  it  is  a  thick  mixture,  rich  in 
food-yelk,  of  albuminous  and  fatty  substances.  As  this 
quantity  of  food-yelk  fills  the  centre  of  the  ovum  before 
cleavage  begins,  there  is  no  difference  in  this  respect 
between  the  mulberry-embryo  and  the  vesicular  embryo. 
The  two  stages,  morula  and  blastula,  rather  agree  in  this. 

When  the  blastula  (Plate  III.,  Fig.  28)  is  fully  formed, 
we  have  again  in  this  case  the  important  folding  or  invagina- 
tion that  determines  gastrulation  (Fig.  29).  At  one  part  of 
the  surface  a  round,  pit-shaped  depression  appears,  and  this 
grows  into  a  cavity — the  primitive  gut-cavity  of  the  gastrula 
(Fig.  29  a);  the  point  of  invagination  forms  the  primitive 
mouth  (0).  The  folded  part  of  the  blastoderm,  the  cells  of 
which  are  enlarged  and  assume  a  slender  cylindrical  shape, 
forms  the  gut-layer  and  encloses  the  primitive  gut-cavity. 
The  superficial  part  of  the  blastoderm  that  is  not  folded 
forms  the  skin-layer  ;  its  cells  become  smaller  by  repeated 
cleavage,  and  are  flattened.  The  space  between  the  skin- 
layer  and  the  gut-layer  (the  remainder  of  the  segmentation- 
cavity)  remains  full  of  food-yelk,  which  is  gradually  used  up. 
This  is  the  only  material  difference  between  our  vesicular 
gastrula  (perigastrula,  Fig.  29)  and  the  original  form  of  the 
bell-gastrula  (archigastrula,  Fig.  6).  Clearly  the  one  has  been 
developed  from  the  other  in  the  course  of  time,  owing  to  the 
accumulation  of  food-yelk  in  the  centre  of  the  ovum.1 

We  must  count  it  an  important  advance  that  we  are  thus 
in  a  position  to  reduce  all  the  various  embryonic  phenomena 
in  the  different  groups  of  animals  to  these  four  principal 
forms  of  segmentation  and  gastrulation.  Of  these  four 
forms  we  must  regard  one  only  as  the  original  palingenetic, 
and  the  other  three  as  cenogenetic  and  derivative.  Both  the 
unequal,  the  discoid,  and  the  superficial  segmentation  have 
clearly  arisen   by  a  secondary  adaptation   from   the   primary 

'  On  the  reduction  of  all  forms  of  gastrulation  (including  " delamination  ") 
to  the  original  palingenetic  form  see  especially  the  lucid  treatment  of  the 
subject  in  Arnold  Lang's  Manual  of  Comparative  Anatomy  (1S8S),  Part  I. 


THE  GASTRULATION  OF  THE  VERTEBRATE  213 

segmentation  j  and  the  chief  cause  o\  their  development  has 
been  the  gradual  formation  of  the  food-yelk,  and  the  increas- 
ing antithesis  between  animal  and  vegetal  halves  of  the  ovum, 
or  between  ectoderm  (skin-layer)  and  entoderm  (gut-layer). 

The  numbers  o(  careful  studies  o(  animal  gastrulation 
that  have  been  made  in  the  last  few  decades  have  completely 
established  the  views  1  have  expounded,  and  which  I  first 
advanced  in  the  years  1N72-76.  For  a  time  they  were 
greatly  disputed  by  many  embryologists.  Some  said  that 
the  original  embryonic  form  of  the  metazoa  was  not  the 
gastrula,  but  the  planula — a  double-walled  vesicle  with 
closed  cavity  and  without  mouth-aperture;  the  latter  was 
supposed  to  pierce  through  gradually.  It  was  afterwards 
shown  that  this  planula  (found  in  several  groups  of  the 
cnidaria)  was  a  later  evolution  from  the  gastrula.  It  was  also 
shown  that  what  is  called  delamination — the  rise  of  the  two 
primary  germinal  layers  by  the  folding  of  the  surface  of  the 
blastoderm  (for  instance,  in  the geryonidce and  other  medusas) 
— was  a  secondary  formation,  due  to  cenogenetic  variations 
in  time,  from  the  original  invagination  of  the  blastula.  The 
same  may  be  said  of  what  is  called  "  immigration,"  in  which 
certain  cells  or  groups  of  cells  are  detached  from  the  simple 
epithelial  layer  of  the  blastoderm,  and  travel  into  the  interior 
of  the  blastula;  they  attach  themselves  to  the  inner  wall  of 
the  blastula,  and  form  a  second  internal  epithelial  layer — that 
is  to  say,  the  entoderm.  In  these  and  many  other  con- 
troversies of  modern  embryology  the  first  requisite  for  clear 
and  natural  explanation  is  a  careful  and  discriminative  dis- 
tinction between  palingenetic  (hereditary)  and  cenogenetic 
(adaptive)  processes.  If  this  is  properly  accomplished,  we 
End  evidence  everywhere  of  the  biogenetic  law. 


FIFTH  TABLE 
PHYLOGENY  OF   VERTEBRATE  GASTRULATION 


Discogastrula  mbl. 
of  the  sauropsida 


Discogastrula  mbi 

of  the  peromel 
(coecilia) 
meroblastic 


Discogastrula  mbi. 

of  the  parasitic 
eyclostoma 
(myxinoida) 


N.  B. 

Hbl.  =holoblastie, 

with  total  cleavage  ; 

Mbl.  =  meroblastic, 

with  partial  cleavage. 


Epigastrula  hbl. 
of  the  viviparous  mamma 
Placenta 


Marsupials 


Discogastrula  mbi. 

of  the  oviparous  mammals 
otrema) 
oblastic 


Amphigastrula  hbl. 

of  most  of  the  amphibU 
holoblastic 


Discogastrula  mbl. 

of  the  bony  fishes 
(teleostii) 


Modern  ganoids 

Discogastrula  mbi. 


Amphigastrula  hbl 

of  the  oldest  fishes 

(early  ganoid 


of  the  modern    selachii 


Early  eyclostoma 
(petromyzoa) 


Arehigastrula  hbl. 

of  the  acrania  (amphioxus) 


SIXTH  TABLE 

SYNOPSIS  OF  THE  FOUR  DIFFERENT  FORMS 
OF  GASTRULATION  OF  THE  VERTEBRATES 


Four  On 
of  Gastrulation. 


Manner  of 
Segmentation. 


Classes  and 
Orders. 


Typical  Genera 
or  Groups. 


I.   First     stage    of  Segmentation 

gastrulation  :  total,  equal  or  un- 

Arehigastrula  equal, 

(bell-gastrula).  Archigastrula. 

Primary     form      of  Ova     very     small, 

the  gastrula.  without     separate 

I'  r  i  in  i  t  i  v  e    u' "  t  food-yelk. 
empty. 


i.  Acrania. 
a  i  Prospondylia. 
1i|  Leptocardia. 


Amphioxus. 
Lancelet. 


i  I     stage    Segmentation    2.  The   older    cy- 


of  gastrulation :  total,  unequal. 

Amphigastrula    Amphigastrula. 
(tufted-gastrula). 

Secondary  form  of    Ova     small,     with 
the  gastrula.         moderate     food- 
Primitive   gut    full     yolk,  telolecithal. 
of  segmented  food- 
yelk. 


Petromyzontes. 
Lampreys. 


clostoma, 
Cydostoma 
hyperoarfia. 

3.  T  h  o    oldest     3a.   Cestracion. 

fishes.  3b.  Accipenser, 

a)  Proselachii.        .50.   Ceratodus. 
hi  Ganoides. 
c)  Dipneusta. 

4.  Most    of     the     4a.  Salamandr, 

amphibia.  4b.  Batrachia. 


III.  Thirdstageof   Segmentation 
gastrulation:  partial,  discoid. 

Diseogastrula.      Discogastrula. 


Tertiary     form     of 
the  gastrula. 

The  embryo  forms 

aflat  or  lens-shaped 
ilisk  which  lios 
above  at  the  ani- 
mal polo  of  the  axis 

of  the  ovum. 
Primitive  gut  with 
large    yelk-sac, 
which  projects  out- 
side the  body. 


Ova    very    large, 

with  plotitx  0  I 
food-yolk,  telole- 
cithal. Tlir 
greater  part  of 
the  food-yelk  is 
n  0 1  segmented, 
and  is  gradually 
absorbed. 


5.  The      parasitic 

cyclostoma, 

Cydostoma 

h  vperotreta. 

b.   Most     of     t  he 

fishes  (exclusive 

0  f  t  h  e  oldest 
s  e  1  a  C  h  i  i  and 
ganoides). 

7.  Peromela 

(gymnophiones). 
s.  Sauropsida 

1  sa  u rop h i d i  a 

and  birds  1. 
9.  T  h  e     oldest 
mammals. 

MullnlrCllld. 


5.     Myxinoides. 


6a.  Sijunlaci'i. 

6b.  Lepidosteus. 

6c.  Teleostei. 

7.  Ccecilia. 

8a.  Reptilia. 

8b.  Arcs. 

qa.  /■'.chid '1111. 

qb.  Ornithorhyn- 
r/l  us. 


IV.   Fourth     stage  Segmentation 

of  gastrulation.  total,  unequal. 

Epigastrula  Epigastrula. 
(mammal  gastrula). 

Quaternary      form  Ova    small,    with 
of  the  gastrula.  atrophied   food- 
Primitive  gut  with  yelk, 
small  yelk-vesicle. 


1.  Mammalia. 
All    living  mam- 
mals, exeept  the 
moiiot  rentes. 
(All  vivipara.) 


10a.   Marsupalia. 
10b.   Placcntiiliu. 


CHAPTER   X. 

THE   CCELOM    THEORY1 

Number  of  the  germinal  layers  in  animals.  Two-layered  and  three-layered 
animals  (coelenteria).  Four-layered  animals,  with  two  limiting  layers  and 
two  eentral  layers  (coelomaria).  Gut-cavity  and  body-cavity.  Nature 
of  the  four  secondary  germinal  layers.  Theories  of  their  origin  (folding' 
and  cleavage).  Older  theories  of  Baer  and  Remak.  Hertwig's  ccelom 
theory  :  formation  of  the  body-cavity,  primarily  by  folding,  secondarily  by 
cleavage.  Approach  of  the  two  coelom-pouches  from  the  primitive  mouth. 
Ccelomation  of  sagitta  and  amphioxus.  Palingenetic  and  cenogenetic 
ccslomation.  Parietal  layer  (skin-fibre  layer)  and  visceral  layer  (gut-fibre 
layer).  Ccelomula  and  chordula.  Corresponding  stem-forms  :  ccelomaea 
and  chordsea.  Separation  of  the  chorda  from  the  dorsal  wall  of  the  primi- 
tive gut  (between  the  two  ccelom-pouches).  Empty  and  full  pouches.  The 
coelom-pouches  of  the  bilaterals  were  originally  sexual  glands.  Their 
ventral  coalescence.  Dorsal  mesentery.  Cenogenetic  ccelomation  of  the 
amphibia  and  amniotes.  The  primitive  mouth  of  the  amniote  embryo 
becomes  the  primitive  groove.  The  border  of  the  primitive  mouth 
(properistoma)  as  vegetation-point  or  source  of  embryonic  development 
(blastocrene).  The  four-layered  coelomula  of  the  reptiles,  birds,  and 
mammals. 

The  two  blastophylls  or  "  primary  germinal  layers  "  which 
the  gastraja  theory  has  shown  to  be  the  first  foundation  in  the 
construction  of  the  body  are  found  in  this  simplest  form 
throughout  life  only  in  ccelenteria  of  the  lowest  grade — in  the 
gastrsads,  olynthus  (the  stem-form  of  the  sponges),  hydra, 
and  cognate  very  simple  cnidaria.  In  all  the  other  animals 
new  strata  of  cells  are  formed  subsequently  between  these 
two  primary  body-layers,  and  these  are  generally  compre- 
hended under  the  title  of  the  middle  layer,  or  mesoderm.  As 
a  rule,  the  various  products  of  this  middle  layer  afterwards 
constitute   the   great   bulk   of  the   animal    frame,   while   the 

1  Cf.  Huxley,  "On  the  Classification  of  the  Animal  Kingdom"  (Quart. 
Journ.  of  Micros.  Sc.,  vol.  xv.);  E.  Ray-Lankester,  "On  the  Invaginate 
Planula  or  Diploblastic  Phase  of  Paludina  Yivipara "  (Quart.  Journ.  of 
Micros.  Sc,  vol.  xv.)  and  "  Revision  of  Speculations  Relative  to  the  Origin  and 
Significance  of  the  Germ-layers"  (Quart.  Journ.  Micros.  Sc,  vol.  xvii. )  ; 
Francis  Balfour,  "  Early  Stages  in  the  Development  of  Vertebrates  "  (Quart. 
Journ.  Micros.  Sc,  vol.  xv. ). 

216 


THE  CCELOM  THEORY  z\- 


original  entoderm,  or  internal  germinal  layer,  is  restricted  to 
the  clothing  of  the  alimentary  canal  and  its  glandular  appen- 
dages ;  and,  on  the  other  hand,  the  ectoderm,  or  external 
germinal  layer,  furnishes  the  outer  clothing  of  the  body,  the 
skin  and  nervous  system. 

In  some  large  groups  of  the  lower  animals  the  middle 
germinal  layer  remains  a  single  connected  mass  ;  these  have 
been  called  the  three-layered  metazoa,  in  opposition  to  the 
two-layered  i;astra?ads  and  hydroids.  To  this  category 
belong,  for  instance,  most  of  the  sponges  and  the  corals  or 
anthozoa.  The  greater  part  of  the  body  in  these  animals 
consists  of  mesodermal  supporting  tissue  and  skeletal  struc- 
tures embedded  therein  ;  the  entodermal  epithelium  confines 
itself  to  clothing  the  alimentary  gastro-canal  system,  the 
ectodermal  epithelium  to  the  cell-coverin£  of  the  outer  skin. 
In  the  platodes  also  (the  spiral,  suctorial,  and  tape  worms) 
the  greater  part  of  the  body  belongs  genetically  to  a  unified 
"  middle  layer,"  which  has  been  developed  between  the  two 
primary  germinal  layers  of  the  gastrula. 

All  these  three-layered  animals  (triploblastica),  like  the 
two-layered  ccelenteria  ( diploblastica  j,  have  no  body-cavity — 
that  is  to  say,  no  cavity  distinct  from  the  alimentary  system  ; 
hence,  they  are  also  called  acoelomia.  On  the  other  hand, 
all  the  higher  animals  have  this  real  body-cavity  ( cocloma ), 
and  so  are  called  ccelomaria.  In  all  these  we  can  distinguish 
four  secondary  germinal  layers,  which  develop  from  the  two 
primary  layers  ;  hence,  the  ccelomaria  may  also  be  contrasted 
with  the  ccelenteria  as  four-layered  metazoa  ( tetrablastica  >. 
To  this  category  belong  all  true  vermalia  (excepting  the 
platodes),  and  also  the  higher  typical  animal  stems  that  have 
been  evolved  from  them — molluscs,  echinoderms,  articulates, 
tunicates,  and  vertebrates. 

The  body-cavity  (caeloma)  is  therefore  a  new  acquisition 
oi  the  animal  body,  much  younger  phylogenetically  than  the 
alimentary  system,  and  of  great  importance  both  morphologi- 
cally and  physiologically.  I  first  pointed  out  this  funda- 
mental significance  of  the  ccelom  in  my  monograph  on  the 
.sponges    (1872),    in    the    section    which    draws    a    distinction 


THE  CCELOM  THEORY 


between  the  body-cavity  and  the  gut-cavity,  and  which 
follows  immediately  on  the  germ-layer  theory  and  the 
ancestral  tree  of  the  animal  kingdom  (the  first  sketch  of  the 
gastraja  theory).  Up  to  that  time  these  two  principal  cavities 
of  the  animal  body  had  been  confused,  or  very  imperfectly 
distinguished  ;  chiefly  because  Leuckart,  the  founder  of  the 
ccelenterata  group  (1848),  has  attributed  a  body-cavity,  but 
not  a  gut-cavity,  to  these  lowest  metazoa.  In  reality,  the 
truth  is  just  the  other  way  about. 

The  ventral  cavity,  the  original  organ  of  nutrition  in  the 
multicellular  animal-body,  is  the  oldest  and  most  important 
organ  of  all  the  metazoa,  and,  together  with  the  primitive 
mouth,  is  formed  in  every  case  in  the  gastrula  as  the  primitive 
gut ;  it  is  only  at  a  much  later  stage  that  the  body-cavity, 
which  is  entirely  wanting  in  the  ccelenterata,  is  developed  in 
some  of  the  metazoa  between  the  ventral  and  the  bodv  wall. 
The  two  cavities  are  entirely  different  in  content  and  purport. 
The  alimentary  cavity  (enteron)  serves  the  purpose  of  diges- 
tion ;  it  contains  water  and  food  taken  from  without,  as  well 
as  the  pulp  (chymus)  formed  from  this  by  digestion.  On  the 
other  hand,  the  body-cavity,  quite  distinct  from  the  gut  and 
closed  externally,  has  nothing  to  do  with  digestion  ;  it 
encloses  the  gut  itself  and  its  glandular  appendages,  and 
also  contains  the  sexual  products  and  a  certain  amount  of 
blood  or  lymph,  a  fluid  that  is  transuded  through  the  ventral 
wall. 

As  soon  as  the  body-cavity  appears,  the  ventral  wall  is 
found  to  be  separated  from  the  enclosing  body-wall,  and  the 
two  continue  to  be  directly  connected  at  various  points.  We 
can  also  then  always  distinguish  a  number  of  different  lavers 
of  tissue  in  both  walls — at  least  two  in  each.  These  tissue- 
layers  are  formed  originally  from  four  different  simple  cell- 
layers,  which  are  the  much-discussed  four  secondary  germinal 
layers.  The  outermost  of  these,  the  skin-sense-layer 
(Figs.  77,  78  /is),  and  the  innermost,  the  gut-gland-layer 
(del),  remain  at  first  simple  epithelia  or  covering-layers. 
The  one  limits  the  outer  surface  of  the  body,  the  other 
the     inner    surface    of    the   ventral    wall  ;    hence    thev   are 


the  ccelom  theory 


called  limiting-layers,  or  methoria.     Between  them  arc   the 

two    middle   layers,   or   mesoblasts,  which   enclose    the  body- 
cavity. 

The  four  secondary  germinal  layers  are  so  distributed  in 
the  structure  of  the  body  in  all  the  coslomaria  (or  all  metazoa 
that  ha\c  a  body-cavity)  that  the  outer  two,  joined  fast 
together,  constitute  the  body-wall,  and  the  inner  two  the 
ventral  wall  ;  the  two  walls  are  separated  by  the  cavity  of  the 
ccelom.  Each  of  the  walls  is  made  up  of  a  limiting  layer 
and  a  middle  layer.  The  two  limiting  layers  chiefly  give  rise 
to  epithelia,  or  covering-tissues,  and  glands  and  nerves, 
while  the  middle  layers  form  the  great  bulk  of  the  fibrous 
tissue,    muscles,    and    connective    matter.      Hence    the    latter 


lu    .  .    ,<<T 


Fig.  77. 


Fig.  78. 


Figs.  77  and  7S.  Diagram  of  the  four  secondary  germinal  layers, 
transverse  section  through  tin-  metazoic  embryo:  Fig.  77  of  an  annelid,  Fig. 
7S  of  a  vermale.  u  primitive  gut,  (/</  ventral  glandular  layer,  rf/ ventral  fibre- 
layer,  hm  skin-fibre-layer,  hs  skin-sense-layer,  »  traces  of  tin-  rudimentary 
kidneys,  '/  trace  of  the  nerve-plates. 


have  also  been  called  fibrous  or  muscular  layers.  The  outer 
middle  lavcr,  which  lies  on  the  inner  side  of  the  skin-sense- 
lavcr,  is  the  skin  fibre-layer;  the  inner  middle  layer,  which 
attaches  from  without  to  the  ventral  glandular-layer,  is  the 
ventral  fibre-layer.  The  former  is  usually  called  briefly  the 
parietal,  and  the  latter  the  visceral  layer,  or  mesoderm.  Of 
the  many  different  names  that  have  been  given  to  the  four 
secondary  germinal  layers,  the  following  are  those  most  in 
use  to-day  : — 


THE  CCELOM  THEORY 


i.  Skin-sense-layer 

(outer  limiting-  layer). 

2.  Skin-fibre-layer 

(outer  middle  layer). 

3.  Gut-fibre-layer 

(inner  middle  layer). 

4-  Gut-gland-layer 

(inner  limiting-  layer). 


I.  Neural  layer 

(neuroblast). 

II.  Parietal  layer 

(myoblast ). 

in.  Visceral  layer 

(gonoblast). 

IV.  Enteral  layer 
(  enteroblast). 


The  two  secondary 

germinal  layers  of  the 

body-wall 

( soma  topi?  it  ra )  : 

I.    Epithelial. 

II.   Fibrous. 

The  two  secondary 

germinal  layers  of  the 

gut-wall 

(splanchnoplfura)  : 

III.  Fibrous. 

IV.  Epithelial. 


The  first  scientist  to  recognise  and  clearly  distinguish  the 
four  secondary  germinal  layers  was  Baer.  It  is  true  that  he 
was  not  quite  clear  as  to  their  origin  and  further  significance, 
and  made  several  mistakes  in  detail  in  explaining  them.  But, 
on  the  whole,  their  great  importance  did  not  escape  him,  and 
he  advanced  the  view  as  to  the  origin  of  the  two  middle 
layers  which  was  afterwards  adopted  by  most  embryologists, 
and  which  I  gave  in  the  first  edition  of  the  Anthropogeny. 
He  derives  each  of  the  middle  layers  separately  from  a 
primary  germinal  layer  (by  cleavage),  and  says  that  the 
outer  or  animal  layer  divides  into  two  folds  (a  skin-layer  and 
a  muscle-layer),  and  the  inner  or  vegetative  layer  into  two 
also  (a  vascular  and  a  mucous  layer).  As  compared  with  the 
more  recent  and  usual  terminology,  Baer's  opinion  may  be 
put  as  follows  : — 


A.  The  two  primary 
germinal  layers  (UastophyUa). 


B.  The  four  secondary  germinal 

layers  (blastoplattce). 


.    1.   Skin-sense-layer  (Baer's  skin-layer). 
I.    Outer  or  animal  germinal  layer    I  Neural  limiting  layer. 

(Skin-layer  Or  ectoderm).        "1    2.   Skin-fibre-layer(Baer  s  muscle-layer). 


II.    Inner  or  vegetative  germinal 

layer 

(gut-layer  or  ectoderm). 


Parietal  middle  layer. 

3.  Gut-fibre-layer  (Baer's  vascular  layer). 

Visceral  middle  laver. 

4.  Gut-gland-layer  (Baer's  mucous  layer). 

Gastral  limiting  layer. 


This  opinion  of  Baer's,  which  had  a  good  deal  of  proba- 
bility in  respect  of  the  physiological  division  of  labour  among 
the  germinal  layers,  had  to  be  given  up  later  on  in  conse- 
quence of  more  accurate  observations.     Remak  had  stated,  in 


THE  ClEI.OM   THEORY 


1850,  in  the  first  part  of  his  distinguished  Studies  of  Verte- 
brate Development,  that  in  the  two-layered  germinal  disk  o( 
the  new-laid  hen's  egg  (our  discogastrulaj  a  few  hours  after 
incubation  the  lower  germinal  layer  divides  into  two — a 
middle  germinal  layer  and  a  glandular  layer.  Subsequently 
the  middle  germinal  layer,  or  fibrous  layer,  had  to  split  up 
again  into  two — an  inner  gut-fibre  layer  and  an  outer  skin- 
fibre  layer.  The  relation  oi'  Remak's  "  three-layer  theory  "  to 
Baer's  original  "tour-layer  theory"  may  be  expressed  as 
follows  : — 

Remak's  three  germinal  layers  The  tour  secondary      The  two  primary 

(three-layer  theory).  germinal  layers  germinal  layers 

(Blast opiates).  of  Baer. 


Outer  or 

'• 

Outer  (or  upper) 

upper 
layer. 

germinal  layer 

1  sensory  layer). 

I. 

Skin-sense-layer 

j     Animal  layer. 

Ectoderm, 

■=-  1 

f      II. 

Middle  germinal 
layer  (motor-ger- 
minative  layer). 

1 

1 

3- 

Skin-fibre-layer 
Gut-fibre-layer 

Skin-layer'. 

Vegetativelay 

layer. 

III. 

Inner  (or  lower) 

.-       Entoderm, 

germinal  layer 

4- 

Gut-gland-layer 

Gut-layer. 

(trophic  layer). 

J 

Remak's  theory  of  the  germinal  layers,  in  the  following-up 
of  which  this  distinguished  observer  made  some  very  impor- 
tant discoveries,  soon  met  with  approval,  especially  as  it  was 
the  first  clear  recognition  of  the  constituent  elementary  parts 
of  the  germinal  layers,  and  the  first  provision  of  an  histo- 
logical foundation  for  ontogeny  by  an  application  of  the  cell 
theory.  The  assumption  that  the  secondary  germinal  layers 
arise  from  the  primary  by  the  cleavage  of  surfaces— in  which 
Baer  and  Remak  agree — was  admitted  by  embryologists  who 
dissented  on  other  points — Kolliker,  for  instance,  who  holds 
that  "in  the  higher  vertebrates  the  middle  germinal  layer 
originates  from  the  outer."  These  generally-accepted 
theories  oi'  cleavage  began  in  give  way  thirty  years  ago, 
when  Kowalevsky  (1871)  showed  that  in  the  ease  of  sagitta  (a 
very  clear  and  typical  subject  of  gastrulation)  the  two  middle 
germinal  layers  and  the  two  limiting  lavers  arise  not  by 
cleavage,  but  by  folding — a  secondary  invagination  oi'  the 
primary  inner  germ-layer.     This  invagination  proceeds  from 


THE  CCELOM  THEORY 


the  primitive  mouth,  at  the  two  sides  of  which  (right  and  left) 
a  couple  of  pouches  are  formed.  As  these  ccelom-pouches  or 
ccelom-sacs  detach  themselves  from  the  primitive  gut,  a 
double  body-cavity  is  formed  (Figs.  77-9). 

The  same  kind  of  ccelom-formation  as  in  sagitta  was  after- 
wards found  by  Kowalevsky  in  brachiopods  and  other 
invertebrates,  and  in  the  lowest  vertebrate — the  amphioxus. 
Further  instances  were  discovered  by  two  English  embryo- 
logists,  to  whom  we  owe  very  considerable  advance  in 
ontogeny — E.  Ray-Lankester  and  F.  Balfour.  On  the 
strength  of  these  and  other  studies,  as  well  as  most  extensive 
research  of  their  own,  the  brothers  Oscar  and  Richard 
Hertwig  constructed  in  1881  the  Ccelom  Theory :  An  Attempt 
to  Explain  the  Middle  Germinal  Layer.  In  order  to  appre- 
ciate fully  the  great  merit  of  this  illuminating  and  helpful 
theory  one  must  remember  what  a  chaos  of  contradictory 
views  was  then  represented  by  the  "  problem  of  the 
mesoderm,"  or  the  much-disputed  "question  of  the  origin 
of  the  middle  germinal  layer."  In  particular  the  curious 
"  parablast  theory"  of  the  Leipzig  embryologist,  His,  based 
on  the  most  perverse  assumptions,  had  caused  a  frightful 
confusion  ;  not  only  all  possible,  but  a  good  many  impossible, 
ideas  as  to  the  origin  of  the  secondary  germinal  layers,  the 
development  of  the  tissues  from  them,  and  the  building-up  of 
the  animal  body,  were  then  seriously  and  dogmatically  dis- 
cussed (cf.  Chapter  III.,  p.  49).  The  ccelom  theory  of  the 
brothers  Hertwig  brought  some  light  and  order  into  this 
infinite    confusion    by    establishing    the    following    points  : 

1.  The  body-cavity  originates  in  the  great  majority  of 
animals  (especially  in  all  the  vertebrates)  in  the  same  way  as 
in  sagitta;  a  couple  of  pouches  or  sacs  are  formed  by  folding 
inwards  at  the  primitive  mouth,  between  the  two  primary 
germinal  layers  ;  as  these  pouches  detach  from  the  primitive 
gut,  a  pair  of  ccelom-sacs  (right  and  left)  are  formed  ;  the 
coalescence  of  these  produces  a  simple  body-cavity  (enteroccel). 

2.  When  these  ccelom-embryos  develop,  not  as  a  pair  of 
hollow  pouches,  but  as  solid  layers  of  cells  (in  the  shape  of 
a    pair  of    mesodermal  streaks) — as    happens    in    the  higher 


Till-:  CCELOM  THEORY 


vertebrates— we  have  a  secondary  (cenogenetic)  modification 
of  the  primary  (palingenetic)  structure  ;  the  two  walls  of 
the  pouches,  inner  and  outer,  are  pressed  together  by  the 
expansion  o\  the  large  food-yelk.  3.  Hence  the  mesoderm 
consists  from  the  first  of  two  genetically  distinct  layers,  which 
do  not  originate  by  the  cleavage  of  a  primary  simple  middle 
layer  (as  Remak  supposed).  4.  These  two  middle  layers 
have,  in  all  vertebrates,  and  the  great  majority  of  the 
invertebrates,  the  same  radical  significance  for  the  construc- 
tion o(  the  animal  body;  the  inner  middle  layer,  or  the 
visceral  mesoderm  (gut-fibre-layer),  attaches  itself  to  the 
original  entoderm,  and  forms  the  fibrous,  muscular,  and 
connective  part  of  the  visceral  wall  (splanchnopleura);  the 
outer  middle  laver,  or  the  parietal  mesoderm  (skin-fibre-layer), 
attaches  itself  to  the  original  ectoderm,  and  forms  the  fibrous, 
muscular,  and  connective  part  of  the  body-wall  ( ' somato- 
pleural J.  5.  It  is  only  at  the  point  of  origination,  the 
primitive  mouth  and  its  vicinity,  that  the  four  secondary 
germinal  layers  are  directly  connected  ;  from  this  point  the 
two  middle  layers  advance  forward  separately  between  the 
two  primary  germinal  layers,  to  which  the}'  severally  attach 
themselves.  6.  The  further  separation  or  differentiation  of 
the  four  secondary  germinal  layers  and  their  division  into  the 
various  tissues  and  organs  take  place  especially  in  the  later 
fore-part  or  head  of  the  embryo,  and  extend  backwards  from 
there  towards  the  primitive  mouth. 

All  animals  in  which  the  body-cavity  demonstrably  arises 
in  this  way  from  the  primitive  gut  (vertebrates,  tunicates, 
echinoderms,  articulates,  and  a  part  of  the  vermalia)  were 
comprised  by  the  Hertwigs  under  the  title  of  cnteroca/a,  and 
were  contrasted  with  the  other  groups  of  the  pseudocosla  (with 
false  body-cavity)  and  the  ccelenterata  (with  no  body-cavity). 
Among  the  pseudoccela  they  counted  the  molluscs  and  a  part 
of  the  vermalia  (plathelmintha,  bryozoa,  and  rotatoria).  In 
these  cases  the  body-cavity  either  represented  a  relic  ot 
the  segmentation-cavity  (blastoccel)  or  arose  secondarily  by 
cleavage  or  the  formation  of  holes  in  a  solid  mesoderm 
(schizocoel).      However,  this  radical  distinction  and  the  views 


THE  CCELOM  THEORY 


as  to  classification  which  it  occasioned  have  been  shown  to 
be  untenable.  Further,  the  absolute  differences  in  tissue- 
formation  which  the  Hertwigs  set  up  between  the  enterocoela 
and  pseudoccela  cannot  be  sustained  in  this  connection.  For 
these  and  other  reasons  their  ccelom-theory  has  been  much 
criticised  and  partly  abandoned.  Nevertheless,  it  has 
rendered  a  great  and  lasting  service  in  the  solution  of  the 
difficult  problem  of  the  mesoderm,  and  a  material  part  of  it 
will  certainly  be  retained.  I  consider  it  an  especial  merit  of 
the  theory  that  it  has  established  the  similarity  of  the 
development  of  the  two  middle  layers  in  all  the  vertebrates, 
and  has  traced  them  as  cenogenetic  modifications  back  to  the 
original  palingenetic  form  of  development  that  we  still  find  in 
the  amphioxus.  Carl  Rabl  comes  to  the  same  conclusion  in 
his  able  Theory  of  the  Mesoderm,  and  so  do  Ray-Lankester, 
Rauber,  Kupffer,  Riikert,  Selenka,  Hatschek,  and  others. 
There  is  a  general  agreement  in  these  and  many  other  recent 
writers  that  all  the  different  forms  of  ccelom-construction,  like 
those  of  gastrulation,  follow  one  and  the  same  strict 
hereditary  law  in  the  vast  vertebrate  stem  ;  in  spite  of  their 
apparent  differences,  they  are  all  only  cenogenetic  modifica- 
tions of  one  palingenetic  type,  and  this  original  type  has 
been  preserved  for  us  down  to  the  present  day  by  the 
invaluable  amphioxus. 

But  before  we  go  into  the  regular  ccelomation  of  the 
amphioxus,  we  will  glance  at  that  of  the  arrow-worm 
(sagitta),  the  remarkable  pelagic  worm  that  is  interesting 
in  so  many  ways  for  comparative  anatomy  and  ontogenv. 
On  the  one  hand,  the  transparency  of  the  clear  body  and  its 
embryo,  and,  on  the  other  hand,  the  typical  simplicity  of  its 
palingenetic  development,  make  the  sagitta  a  most  instructive 
object  in  connection  with  various  problems.  The  class  of 
the  chcetognatlia,  which  is  only  represented  by  the  cognate 
genera  of  sagitta  and  spadella,  is  in  another  respect  also  a 
most  remarkable  branch  of  the  extensive  worm-stem.  It  was 
therefore  very  gratifying  that  Oscar  Hertwig  (1880)  fully 
explained  the  anatomy,  classification,  and  evolution  of  the 
chajtognatha  in  his  careful  monograph. 


THE  CCELOM  THEORY 


The  spherical  blastula  thai  arises  from  the  impregnated 
ovum  o(  the  sagitta  is  converted  by  uni-polar  folding  into  a 
typical  archigastrula,  entirely  similar  to  that  of  the  monoxenia 
which  I  described  (Chapter  VIII.,  Fig.  ,}i).  This  oval, 
uni-axial  cup-larva  (circular  in  section)  becomes  bilateral 
(or  tri-axial)  by  the  growth  o(  a  couple  of  ccelum-pouches 
from  the  primitive  gut  (Figs.  79,  80).  To  the  right  and  left 
a  sac-shaped  fold  appears  towards  the  oral  pole  (where  the 
permanent  month,  in,  afterwards  arises).  The  two  sacs 
are  at  fust  separated  by  a  couple  oi  folds  of  the  entoderm 
(Fig.  71)  pv),  and  are  still   connected  with   the  primitive  gut 


Fig. 


:•>■ 


Fig.  So. 


Fig.  7.).  Ccelomula  of  sagitta  (ic.-isirula  with  a  couple  of  coelom-pouches). 
(From  kowalevsky.)  bUp  primitive  mouth,  al  primitive  gut,  frv  ccelom-folds, 
m  permanent  mouth. 

'.  -Ccelomulaof  sagitta.  in  soil  ion.  CFroxaHertiuig.)  D  dorsal  side, 
V ventral  sido,  ik  inner  germinal  layer,  mv  visceral  mesoblast,  //;  body-cavity, 
mp  parietal  mesoblast,  ak  outer  germinal  layer. 


by  wide  apertures;  they  also  communicate  for  a  short  time 
with  the  dorsal  side  (Fig.  80  </).  Soon,  however,  the  ccelom- 
pouches  completely  separate  from  each  other  and  from  the 
primitive  gut;  at  the  same  time  they  enlarge  so  much  that 
they  close  round  the  primitive  gut  (Fig.  81).  But  in  the 
middle  line  o(  the  dorsal  and  ventral  sides  the  pouches 
remain  separated,  their  approaching  walls  joining  here  to 
form  a  thin  vertical  partition,  the  mesentery  (dm  and  vm  j. 
Thus  sagitta  has  throughout  life  a  double  body-cavity 
(Fig.   81   ///),  and   the   gut   is   fastened  to   the   body-wall  both 

Q 


226 


THE  CCELOM  THEORY 


above  and  below  by  a  mesentery — below  by  the  ventral 
mesentery  fvmj,  and  above  by  the  dorsal  mesentery  (dm). 
The  inner  layer  of  the  two  ccelom-pouches  (visceral  meso- 
blast,  mv)  attaches  itself  to  the  entoderm  ( ik),  and  forms 
with  it  the  visceral  wall.  The  outer  layer  (parietal  meso- 
blast,  nip)  attaches  itself  to  the  ectoderm  (ak),  and  forms 
with  it  the  outer  body  wall.  Thus  we  have  in  sagitta  a 
perfectly  clear  and  simple  illustration  of  the  original 
ccelomation  of  the  enteroccela.  This  palingenetic  fact  is  the 
more  important,  as  the  greater  part  of  the  two  body-cavities 
in  sagitta  changes  afterwards  into  sexual  glands — the  fore  or 
female  part  into  a  pair  of  ovaries,  and  the  hind  or  male  part 
into  a  pair  of  testicles. 

Ccelomation  takes  place  with  equal 
clearness  and  transparency  in  the  case 
of  the  amphioxus,  the  lowest  vertebrate, 
and  its  nearest  relatives,  the  inverte- 
brate tunicates,  the  ascidia.  However, 
in  these  two  stems,  which  we  class 
together  as  chordonia,  this  important 
process  is  more  complex  as  two  other 
processes  are  associated  with  it — the 
development  of  the  chorda  from  the 
entoderm  and  the  separation  of  the 
medullary  plate  or  nervous  centre  from 
the  ectoderm.  Here  again  the  s'cull- 
less  amphioxus  has  preserved  to  our 
own  time  by  tenacious  heredity  the  chief 
phenomena  in  their  original  form,  while  it  has  been  more  or 
less  modified  by  embryonic  adaptation  in  all  the  other  verte- 
brates (with  skulls).  Hence  we  must  once  more  thoroughly 
understand  the  palingenetic  embryonic  features  of  the 
lancelet  before  we  go  on  to  consider  the  cenogenetic  forms  of 
the  craniota. 

The  ccelomation  of  the  amphioxus,  which  was  first 
observed  by  Kowalevsky  in  1867,  has  been  very  carefully 
studied  since  by  Hatschek  (18S1).  According  to  him,  there 
are   first    formed   on   the   bilateral    gastrula  we   have  already 


Fig.  81.— Section  of  a 
young  sagitta.  (From 
Hertwig.  1  dh  visceral- 
cavity,  ik  andai  inner  and 
outer  limiting  layers,  mv 
and  nip  inner  and  outer 
middle  layers,  III  body- 
cavity,  dm  and  vm  dorsal 
and  visceral  mesentery. 


THE  CCELOM  THEORY 


considered  (Figs.  40,  41)  three  parallel  longitudinal  folds — 
one  single  ectodermal  fold  in  the  central  line  of  the  dorsal 
surface,  and  a  pair  o(  entodermie  folds  at  the  two  sides  of  the 
former.  The  broad  ectodermal  fold  that  first  appears  in  the 
medium  line  o(  the  flattened  dorsal  surface,  and  forms  a 
shallow  longitudinal  groove,  is  the  beginning  of  the  central 
nervous  system,  the  medullary  tube.  Thus  the  primary 
outer  germinal  layer  divides  into  two  parts,  the  medium 
medullary  plate  (Fig.  84  nip)  and  the  horn-plate  ( ' ak J,  the 
beginning  o(  the  outer  skin  or  epidermis.  As  the  parallel 
borders  of  the  concave   medullary   plate    fold    towards   each 


Fig.  8j 


Figs.  s.>  and  83.—  Transverse  section  of  amphioxus-larvae.  (From 
Haischek.)  Fig.  82  at  the  commencement  of  ccelom-formation  (siill  without 
segments),  Fig.  83  at  the  stage  with  lour  primitive  segments,  til-,  ik,  »ik  outer, 
inner,  and  middle  germinal  layer,  lip  horn  plate,  mp  medullary  plate,  ch  chorda, 
*  ana     disposition  of  the  ccelom-pouches,  //;  body-cavity. 


other  and  ijrow  underneath  the  horn-plate,  a  cylindrical  tube 
is  formed,  the  medullary  tube  (Fig.  85  n);  this  quickly 
detaches  itself  altogether  from  the  horn-plate.  At  each  side 
of  the  medullary  tube,  between  it  and  the  alimentary  tube 
(l;ii,rs.  S2  S5  <///),  the  two  parallel  longitudinal  folds  grow  out 
o(  the  dorsal  wall  of  the  alimentary  tube,  and  these  form  the 
two  ccelom-pouches  (Figs.  83  and  S4  //;).  This  part  of  the 
entoderm,  which  thus  represents  the  first  structure  of  the 
middle  germinal  layer,  is  shown  darker  than  the  rest  o(  the 
inner  germinal  layer  in  Figs.  82  85.  The  place  of  the 
double  mesoderm ie  fold   is  indicated    in  Fig.  83  with  asterisks 


THE  CCELOM  THEORY 


(*  *).  The  basal  edges  of  the  curved  folds  grow  together 
at  these  points,  and  form  closed  pouches  (Fig.  84  in  trans- 
verse section).  The  hindermost  part  of  the  two  parallel 
mesodermic  folds  attaches  originally  to  the  border  of  the 
primitive  mouth,  and  is  connected  there  with  the  two  large 
"  primitive  mesodermic  cells"  or  "  promesoblasts,"  which  we 
have  considered  previously  (Fig.  41  p).  The  embryonic 
structures  that  develop  from  the  latter  may  be  called,  with 
Rabl,  peristomal  mesoblasts,  in  opposition  to  the  structures 
of  the  former,  the  gastral  mesoblasts. 

During  this  interesting  process  the  outline  of  a  third  very 
important  organ,  the  chorda  or  axial  rod,  is  being  formed 
between  the- two  ecelom-pouches.     This  first    foundation   of 


Fig.  85. 


Figs.  84  and  85.— Transverse  section  of  amphioxus  embryo.  Fig.  S4 
at  the  stage  with  five  somites.  Fig.  85  at  the  stage  with  eleven  somites.  (From 
Hatschei.)  ak  outer  germinal  layer,  mp  medullary  plate,  n  nerve-tube,  ik  inner 
germinal  layer,  d/i  visceral  cavity,  //;  body-cavity,  ink  middle  germinal  layer 
(mkl  parietal,  ///k.:  visceral),  us  primitive  segment,  c/i  chorda. 

the  skeleton,  a  solid  cylindrical  cartilaginous  rod,  is  formed 
in  the  median  line  of  the  dorsal  primitive  gut-wall,  from  the 
entodermal  cell-streak  that  remains  here  between  the  two 
ecelom-pouches  (Figs.  S2-85  ch).  The  chorda  appears  at  first 
in  the  shape  of  a  flat  longitudinal  fold  or  a  shallow  groove 
(Figs.  83,  84) ;  it  does  not  become  a  solid  cylindrical  cord 
until  after  separation  from  the  primitive  gut  (Fig.  85). 
Hence  we  might  say  that  the  dorsal  wall  of  the  primitive  gut 
forms  three  parallel  longitudinal  folds  at  this  important 
period — one  single  and  a  pair  of  folds.  The  single  medium 
longitudinal  fold  becomes  the  chorda,  and  lies  immediately 


THE  CCELOM  THEORY 


below  the  middle  longitudinal  groove  o\  the  ectoderm,  which 

bocomo  the  medullary  tube  ;  the  pair  of  longitudinal  folds, 
right  and  left,  lie  at  the  sides  between  the  former  and  the 
latter,  and  form  the  ccelom-pouches.  The  part  of  the 
primitive  gut  that  remains  after  the  cutting  o(i  o(  these  three 
dorsal  primitive  organs  is  the  permanent  gut  (enteron  or 
mesodaeum);  its  entoderm  is  the  gut-gland-layer  or  enteric 
layer  (enteroblast). 

I  give  the  name  of  ckordula  or  chordalarva  to  the 
embryonic  stage  o(  the  vertebrate  organism  which  is  repre- 
sented by  the  amphioxus  larva  at  this  period  (Figs.  86,  87,  in 
the  third  period  of  development  according  to  Hatschek). 
(Strabo  and  Plinius  give  the  name  of  cordula  or  cordyla  to 
young  tish  larvae.)  I  ascribe  the  utmost  phylogenetic  signi- 
ficance to  it,  as  it  is  found  in  all  the  chordoma  (tunicates  as 
well  as  vertebrates)  in  essentiallv  the  same  form.  Although 
the  construction  of  the  large  food-yelk  greatly  modifies  the 
form  of  the  chordula  in  the  higher  vertebrates,  it  remains  the 
same  in  its  main  features  throughout.  In  all  cases  the  nerve- 
tube  '  m  >  lies  on  the  dorsal  side  of  the  bilateral,  worm-like 
body,  the  visceral  tube  fdj  on  the  ventral  side,  the  chorda 
fchj  between  the  two,  on  the  long  axis,  and  the  ccelom- 
pouches  (c)  at  each  side.  In  every  case  these  primitive 
organs  develop  in  the  same  way  from  the  germinal  layers, 
and  the  same  organs  always  arise  from  them  in  the  mature 
chorda-animal.  Hence  we  may  conclude,  according  to  the 
laws  of  heredity  of  the  theory  of  descent,  that  all  these 
chordonia  or  chordata  (tunicates  and  vertebrates)  descend 
from  an  ancient  common  ancestral  form,  which  we  may  call 
chordcea.  We  should  regard  this  long-extinct  chordasa,  if  it 
were  still  in  existence,  as  a  special  class  of  unarticulated 
worm  (chordariaj.  It  is  especially  noteworthy  that  neither 
the  dorsal  nerve-tube  nor  the  ventral  gut-tube,  nor  even  the 
chorda  that  lies  between  them,  shows  any  trace  of  articulation 
or  metamera-formation  ;  even  the  two  ccelom-sacs  are  not 
segmented  at  first  (though  in  the  amphioxus  they  quickly 
divide  into  a  series  of  somites  by  transverse  folding).  These 
ontogenetic    facts   are    of    the    greatest    importance     for    the 


THE  CCELOM  THEORY 


purpose  of  learning  those  ancestral  forms  of  the  vertebrates 
which  we  have  to  seek  in  the  group  of  the  unarticulated 
vermalia.  The  ccelom-pouches  were  originally  sexual 
glands  in  these  ancient  chordonia. 

From  the  phylogenetic  point  of  view  the  ccelom-pouches 


Fig.  87. 


o      h    b         d  z  dd  ch  ti"  m  n'  h    ch 
Fig.  88. 


Fig.  89. 


Figs.  86  and  87.— Chordula  Of  the  amphioxus.  Fig.  86  median  longi- 
tudinal section  (seen  from  the  left).  Fig.  87  transverse  section.  1  From 
Hatschek.)  In  F"ig.  86  the  ccelom-pouches  are  omitted,  in  order  to  show  the 
chordula  more  clearly.  F~ig.  87  is  rather  diagrammatic.  /;  horn-plate,  m 
medullary  tube,  //  wall  of  same  (»'  dorsal  n"  ventral),  ch  chorda,  np  neuroporus, 
tie  canalis  neurentericus,  d  gut-cavity,  r  gut  dorsal  wall,  b  gut  ventral  wall, 
s  yelk-cells  in  the  latter,  11  primitive  mouth,  0  mouth-pit,  p  promesoblasts 
(primitive  or  polar  cells  of  the  mesoderm),  ti'  parietal  layer,  ?'  visceral  layer  oi 
the  mesoderm,  c  ccelom,  f  rest  of  the  segmentation-cavity. 

Figs.  S8  and  89.—  Chordula  of  the  amphibia  (the  ringed  snake).  (From 
Goette.)  Fig.  88  median  longitudinal  section  (seen  from  the  left),  Fig.  89 
transverse  section  (slightly  diagrammatic).     Lettering  as  in  Figs.  86  and  87. 


THE  CCELOM  THEORY 


arc,  in  any  case,  older  than  the  chorda  ;  since  they  also  develop 
in  the  same  way  as  in  the  chordoma  in  a  number  of  inverte- 
brates which  have  no  chorda  (for  instance,  sagitta,  Figs. 
7q  Si).  Moreover,  in  the  amphioxus  the  first  outline  of  the 
chorda  appears  later  than  that  of  the  ccelom-sacs.  Hence  we 
must,  according  to  the  biogenetic  law,  postulate  a  special 
intermediate  form  between  the  gastrula  and  the  chordula, 
which  we  will  call  cceiomula,  an  unarticulated,  worm-like 
body  with  primitive  gut,  primitive  mouth,  and  a  douhle 
body-cavity,  hut  no  chorda.  This  embryonic  form,  the 
bilateral  cceiomula  (Fig.  84),  may  in  turn  be  regarded  as  the 
ontogenetic  reproduction  (maintained  by  heredity)  of  an 
ancient  ancestral  form  of  the  civlomaria,  the  ccelomaea  (cf. 
Chapter  XX. ). 

In  sagitta  and  other  helmintha  the  two  ccelom-pouches 
(presumably  the  gonades  or  sex-glands  of  the  ceeloma?a)  are 
separated  by  a  complete  median  partition,  the  dorsal  and 
ventral  mesentery  (Fig.  Si  dm  and  vm) ;  but  in  the 
vertebrates  only  the  upper  part  of  this  vertical  partition  is 
maintained,  and  forms  the  dorsal  mesenterv.  This  mesenterv 
afterwards  takes  the  form  of  a  thin  membrane,  which  fastens 
the  visceral  tube  to  the  chorda  (or  the  vertebral  column). 
At  the  under  side  of  the  visceral  tube  the  ccelom-sacs  blend 
together,  their  inner  or  median  walls  breaking  down  and 
disappearing.  The  body-cavity  then  forms  a  single  simple 
hollow,  in  which  the  gut  is  quite  free,  or  only  attached  to  the 
dorsal  wall  by  means  of  the  mesentery  (cf.  Plate  IY.,  Fig.  5). 

The  development  of  the  body-cavity  and  the  formation  of 
the  chordula  in  the  higher  vertebrates  is,  like  that  of  the 
gastrula,  chiefly  modified  by  the  pressure  of  the  food-yelk  on 
the  embryonic  structures,  which  forces  its  hinder  part  into  a 
discoid  expansion.  These  cenogenetic  modifications  seem 
to  be  so  great  that  until  twenty  years  ago  these  important 
processes  were  totally  misunderstood.  It  was  generally 
believed  that  the  body-cavity  in  man  and  the  higher 
vertebrates  was  due  to  the  division  of  a  simple  middle 
layer,  and  that  the  latter  arose  by  cleavage  from  one  or  both 
of  the   primary  germinal   layers.      The  truth  was  brought  to 


THE  CCELOM  THEORY 


light  at  last  by  the  comparative  embrvological  research  of 
the  Hertwigs.  They  showed  in  their  Coelom  Theory  (1881) 
that  all  vertebrates  are  true  enteroccela,  and  that  in  every 
case  a  pair  of  ccelom-pouches  are  developed  from  the 
primitive  gut  by  folding.  The  cenogenetic  chordula-forms 
of  the  craniotes  must  therefore  be  derived  in  the  same  way 
from  the  palingenetic  embryology  of  the  amphioxus,  as  I 
had  previously  proved  for  their  gastrula-forms. 

The  chief  difference  between  the  ccelomation  of  the 
acrania  (amphioxus J  and  the  other  vertebrates  (craniotes) 
is  that  the  two  ccelom-folds  of  the  primitive  gut  in  the 
former  are  from  the  first  hollow  vesicles,  filled  with  fluid,  but 
in  the  latter  are  empty  pouches,  the  layers  of  which  (inner 
and  outer)  close  with  each  other.  In  common  parlance  we 
still  call  a  pouch  or  pocket  by  that  name,  whether  it  is  full 
or  empty.  It  is  different  in  ontogeny;  in  embrvological 
literature  ordinary  logic  does  not  count  for  very  much.  In 
many  of  the  manuals  and  large  treatises  on  this  science  it 
is  proved  that  vesicles,  pouches,  or  sacs  deserve  that  name 
only  when  they  are  inflated  and  filled  with  a  clear  fluid. 
When  they  are  not  so  filled  (for  instance,  when  the  primitive 
gut  of  the  gastrula  is  filled  with  yelk,  or  when  the  walls 
of  the  empty  coelom-pouches  are  pressed  together),  these 
vesicles  must  not  be  cavities  any  longer,  but  "  solid 
structures." 

The  evolution  of  the  large  food-yelk  in  the  ventral  wall 
of  the  primitive  gut  (Figs.  88,  89)  is  the  simple  cenogenetic 
cause  that  converts  the  sac-shaped  ccelom-pouches  of  the 
acrania  into  the  leaf-shaped  ccelom-streaks  of  the  craniotes. 
To  convince  ourselves  of  this  we  need  only  compare,  with 
Hertwig,  the  palingenetic  ccelomula  of  the  amphioxus 
(Figs.  83,  84)  with  the  corresponding  cenogenetic  form  of  the 
amphibia  (Figs.  92-94),  and  construct  the  simple  diagram 
that  connects  the  two  (Figs.  90,  91).  If  we  imagine  the 
ventral  half  of  the  primitive  gut-wall  in  the  amphioxus 
embryo  (Figs.  82-87)  distended  with  food-yelk,  the  vesicular 
ccelom-pouches  (lh)  must  be  pressed  together  by  this,  and 
forced  to  extend  in  the  shape  of  a  thin  double  plate  between 


THE  CCELOM  THEORY 

the  gut-wall  and  body-wall  (Figs.  89,  cp^TaasUx^hsTorf 
follows  a  downward  and  forward  direction.  They  arc  not 
directly  connected  with  these  two  walls.  The  real  unbroken 
connection  between  the  two  middle  layers  and  the  primary 
germ-layers  is  found  right  at  the  back,  in  the  region  of  the 
primitive  mouth  (Fig.  qo  11).  At  this  important  spot  we 
have  the  source  of  embryonic  development  (blastocrene),  or 
"zone  of  growth,"  from  which  the  coelomation  (and  also  the 
gastrulation)  originally  proceeds. 

Hertwig  even    succeeded    in    showing,   in   the    ceelomula- 
embrvo  of  the  water  salamander  ( ' Iriton J,  between   the  first 


Fig.  00. 


91. 


Figs.  9oand  91.  Diagrammatic  vertical  section  of  ecelomula-embryos 
Of  vertebrates.  (From  Hertwig.}  Fig.  90,  vertical  section  through  the 
primitive  mouth.  Fig.  91,  vertical  section  before  the  primitive  mouth. 
11  primitive  mouth,  net  primitive  gut,  d  yelk,  ilk  yelk-nuclei,  dh  gut-cavity, 
///  body-cavity,  w/>  medullary  plate,  ch  chorda  plate,  uk  and  ik  outer  and  inner 
germinal  layers,  pb  parietal  ami  -.■!>  visceral  mesoblast. 

structures  of  the  two  middle  layers,  the  relic  of  the  body- 
cavity,  which  is  represented  in  the  diagrammatic  transitional 
form  (Figs.  90,  91).  In  sections  both  through  the  primitive 
mouth  itself  (Fig.  92)  and  in  front  of  it  (Fig.  93)  the  two 
middle  layers  (pb  and  vb )  diverge  from  each  other,  and 
disclose  the  two  body-cavities  as  narrow  clefts.  At  the 
primitive  mouth  itself  (Fig.  <>;,  u)  we  can  penetrate  into  them 
from  without.  It  is  only  here  at  the  border  oi  the  primitive 
mouth  that  we  can  show  the  direct  transition  of  the  two  middle 
layers  into  the  two  limiting  lasers  or  primary  germinal  layers. 


THE  CCELOM  THEORY 


The  structure  of  the  chorda  also  shows  the  same  features 
in  these  ccelomula-embryos  of  the  amphibia  (Fig.  94)  as  in 
the  amphioxus  (Figs.  82-85).  It  arises  from  the  entodermic 
cell-streak,  which  forms  the  middle  dorsal  line  of  the 
primitive  gut,  and  occupies  the  space  between  the  flat 
ccelom-pouches  (Fig.  94  A).  While  the  nervous  centre  is 
formed  here  in  the  median  line  of  the  back  and  separated 
from  the  ectoderm  as  "  medullary  tube,"  there  takes  place  at 
the  same  time,  directly  underneath,  the  severance  of  the 
chorda  from  the  entoderm  (Fig.  94,  A,  B,  C).  Under  the 
chorda  is  formed  (out  of  the  ventral  entodermic  half  of  the 


Fig.  92. 


Fig.  93. 


Figs.  92  and  93.— Transverse  section  of  ecelomula  embryos  of  triton. 
(From  Hertwig. )  Fig-.  92  .section  through  the  primitive  mouth,  Fig.  93  section 
in  front  of  the  primitive  mouth.  11  primitive  mouth,  dh  gut-cavity,  dz  yelk-cells, 
d  /"yelk-stopper,  ak  outer  and  ik  inner  germinal  layer,/*  parietal  and  vb  visceral 
middle  layer,  m  medullary  plate,  rh  chorda. 


gastrula)  the  permanent  gut  or  visceral  cavity  (enteron) 
(Fig.  94,  B,  d/i).  This  is  done  by  the  coalescence,  under  the 
chorda  in  the  median  line,  of  the  two  dorsal  side-borders  of 
the  gut-gland-layer  ( ik),  which  were  previously  separated  by 
the  chorda-plate  (Fig.  94,  A,  c/t);  these  now  alone  form  the 
clothing  of  the  visceral  cavity  (dh  )  (enteroderm,  Fig.  94,  C). 
All  these  important  modifications  take  place  at  first  in  the 
fore  or  head-part  of  the  embryo,  and  spread  backwards  from 
there  ;  here  at  the  hinder  end,  the  region  of  the  primitive 
mouth,  the  important  border  of  the  mouth  (or  properistoma) 


THE  CCE/.O.U  THEORY 


*3S 


remains  for  a  long  time  the  source  of  development  (blasto- 
crenej,  or  the  zone  of  fresh  construction,  in  the  further 
building-up  of  the  organism. 

One  has  only  to  compare  carefully  the  illustrations  given 
(Figs.  88  <»4)  to  see  that,  as  a  fact,  the  cenogenetic  coelomation 
of  the  amphibia  can  be  deduced  directly  from  the  palingenetic 


.!./..<.  Vertical  section  of  the  dorsal  part  of  three  triton- 
embryos.  (From  Hertvig.)  In  Fig.  .1  tlx-  medullary  swellings  (the  parallel 
borders  of  the  medullary  plate)  begin  to  rise  ;  in  Fig.  11  they  grow  towards 
each  other;  in  Fig.  C  they  join  and  form  the  medullary  tube.  >»/>  medullary 
plate,  m/medullary  folds,  n  nerve-tube,  ch  chorda,  iM  body-cavity,  mk,  and  mk., 
parietal  and  visceral  mesoblasts,  uv  primitive-segmeni  cavities,  ai  ectoderm, 
it  entoderm,  </-  yelk-cells,  dh  gut-cavity. 


236  THE  CCELOM  THEORY 

form  of  the  acrania  (Figs.  82-87).  Hence  Hertwig  was  quite 
right  in  formulating  the  following  important  thesis  on  the 
basis  of  this  comparison  :  "  The  closing  of  the  permanent 
gut  at  the  dorsal  side,  the  severance  of  the  two  body-sacs 
from  the  inner  germinal  layer,  and  the  rise  of  the  chorda 
dorsalis,  are  processes  with  the  most  intimate  relations  to 
each  other,  both  in  the  amphibia  and  the  amphioxus.  Here 
also  the  severance  of  the  said  parts  begins  at  the  head- 
extremity  of  the  embryo,  and  proceeds  slowly  backwards, 
where  for  a  long  time  a  zone  of  new  formation  remains,  by 
means  of  which  the  longitudinal  growth  of  the  body  is 
effected." 

The  same  principle  holds  good  for  the  amniotes,  the  three 
higher  classes  of  vertebrates,  although  in  this  case  the  pro- 
cesses of  ccelomation  are  more  modified  and  more  difficult  to 


Fig.  95.— Transverse  section  of  the  ehordula-embryo  of  a  bird  (from 

a  lien's  egg  at  the  close  of  the  first  day  of  incubation).  (From  KSUiker.) 
h  horn-plate  (ectoderm),  m  medullary  plate,  vP/'dorsal  folds  of  same,  Pv  medullary 
furrow,  eh  chorda,  UTVp  median  (inner)  part  of  the  middle  laver  (median  wall  of 
the  ccelom-pouches),  sp  lateral  (outer)  part  of  same,  or  lateral  plates,  ipmh 
structure  of  the  body-cavity,  dd  gut-gland-layer. 

identify  on  account  of  the  colossal  accumulation  of  food-yelk 
and  the  corresponding  notable  flattening  of  the  germinal 
disk.  However,  as  the  whole  group  of  the  amniotes  has 
been  developed  at  a  comparatively  late  date  from  the  class  of 
the  amphibia,  their  coelomation  must  also  be  directly  trace- 
able to  that  of  the  latter.  This  is  really  possible  as  a  matter 
of  fact ;  even  the  older  and  more  objective  illustrations 
showed  an  essential  identity  of  features.  Thus  forty  years 
ago  Kolliker  gave,  in  the  first  edition  of  his  Evolution  of 
Man  (1861),  some  sections  of  the  chicken-embryo,  the  features 
of  which  could  at  once  be  reduced  to  those  already  described 
and  explained  in  the  sense  of  Hertwig's  ccelom-theory.  A 
section  through  the  embryo  of  the  hatched  hen's  egg  towards 
the  close  of  the  first  day  of  incubation  shows  in  the  middle 


THE  CCELOM  THEORY 


of  the  dorsal  surface  a  broad  ectodermic  medullary  groove 
(Fig.  <)>  A'  /).  and  underneath  the  middle  of  the  chorda  (  c// ) 
and  at  each  side  of  it  a  couple  of  broad  mesodermic  layers 
(sp).  These  enclose  a  narrow  space  or  cleft  (uwh),  which 
is  nothing  else  than  the  structure  of  the  body-cavity.  The 
two  layers  that  enclose  it — the  upper  parietal  layer  ( '  Iipl ' )  and 
the  lower  visceral  layer  (df) — are  pressed  together  from 
without,  hut  clearly  distinguishable.  This  is  even  clearer  a 
little  later,  when  the  medullar)-  furrow  is  closed  into  the 
nerve-tube  (Fig.  96  mr).  Here  the  mesoderm  has  divided 
into  two  sections  by  a  longitudinal  fold,  an  inner  (median) 
primitive-segment  plate  ( 'una )  and  an  outer  (Lateral)  plate; 
the  narrow-  ccelom-eleft  may  be  seen  both  in  the  former 
(uwh)  and    the    latter    (nip).      It    afterwards    enlarges    into 


-  "V    .J'P? 


el.  Or*        ao        *'p        </</      df 

Fig.  go.  —Transverse  section  of  the  vertebrate-embryo  of  a  bird 
(from  a  lien's  egg  on  the  second  day  of  incubation  I.  (  From  KSUiker. )  It  horn- 
plate,  tnr  medullary  tube,  ch  chorda,  //:.■  primitive  segments,  wwh  primitive 
segment  cavity  (median  relic  of  the  caelom),  sp  lateral  ccelom-cleft,  hpl  skin- 
fibre-layer,  df  gut-fibre-layer,  ung  primitive-kidney  pa^si^-,  an  primitive  aorta, 
tUI  gut-gland-layer. 

the  secondary  body-cavity,  the  parietal  skin-fibre-layer  ( hpl ) 
and  the  visceral  gut-fibre-layer  (df)  blending  together. 

In  this  special  importance  attaches  to  the  fact  that  here 
again  the  four  secondary  germinal  layers  are  already  sharply 
distinct,  and  easily  separated  from  each  other.  There  is  only 
one  very  restricted  area  in  which  they  are  connected,  and 
actually  pass  into  each  other ;  this  is  the  region  o(  the 
primitive  mouth,  which  is  contracted  in  the  amniotes  into  a 
dorsal  longitudinal  cleft,  the  primitive  groove.  Its  two 
lateral  lip-borders  form  the  primitive  streak,  which  has  long 
been  recognised  as  the  mosl  important  embryonic  source  and 
starting-point  of  further  processes  ( Remak's  "  axial  plate"). 
Sections  through  this  primitive  streak  (Figs.  <)J  and  98)  show 
that  the  two  primary  germinal   layers  grow  at   an   early  stage 


THE  CCELOM  THEORY 


(in  the  discogastrula  of  the  chick,  a  few  hours  after  incuba- 
tion) into  the  primitive  streak  (x),  and  that  the  two  middle 
layers  extend  outward  from  this  thickened  axial  plate  (y )  to 
the  right  and  left  between  the  former.  The  plates  of  the 
ccelom-layers,  the  parietal  skin-fibre-layer  (m)  and  the 
visceral  gut-fibre-layer  (~f),  are  seen  to  be  still  pressed  close 
together,  and  only  diverge  later  to  form  the  body-cavity. 
Between  the  inner  (median)  borders  of  the  two  flat  coelom- 
pouches  lies  the  chorda  (Fig.  98,  .v),  which  here  again 
developes  from  the  middle  line  of  the  dorsal  wall  of  the 
primitive  gut. 

y  If  A  T  1 


Figs.  97  and  98.— Transverse  section  of  the  primitive  streak  (primi- 
tive mouth)  of  the  ehiek.  Fig.  97  a  few  hour*  alter  the  commencement  of 
incubation,  Fig.  98  a  little  later.  (From  Waldeyer.)  h  horn-plate,  n  nerve- 
plate,  111  skin-fibre  layer,  /"  gut-fibre-layer,  d  gut-gland-layer,  y  primitive  streak 
or  axial  plate,  in  which  all  lour  germinal  layers  meet,  x  structure  of  the  chorda, 
11  region  of  the  later  primitive  kidneys. 


Ccelomation  takes  place  in  the  vertebrates  in  just  the 
same  way  as  in  the  birds  and  reptiles.  This  was  to  be 
expected,  as  the  characteristic  gastrulation  of  the  mammal 
has  descended  phylogenetically  from  that  of  the  reptiles.  In 
both  cases  a  discogastrula  with  primitive  streak  arises  from 
the  segmented  ovum,  a  two-layered  germinal  disk  with  long 
and  small  hinder  primitive  mouth.  Here  again  the  two 
primary  germinal  layers  are  only  directly  connected 
(Fig.  99  pr)  along  the  primitive  streak  (at  the  invagination- 
point  of  the  blastula),  and  from  this  spot  (from  the  pro- 
peristoma   or   border   of    the    primitive    mouth)   the    middle 


THE  CCELOM  THEORY 


germinal  layers  mk)  grow  out  to  right  and  left  between  the 
preceding.  In  the  fine  illustration  of  the  ccelomula  o(  the 
hare  which  Van  Beneden  has  given  us  (Fig.  99)  one  can 
clearly  sec  that  each  of  the  four  secondary  germinal  layers 
consists  of  a  single  stratum  of  cells. 

Finally,  we  must  point  out,  as  a  fact  of  the  utmost  impor- 
tance for  our  anthropogeny  and  of  great  general  interest,  that 
the  four-layered  ccelomula  of  man  lias  just  the  same  construc- 
tion as  that  of  the  hare  (Fig.  99).  A  vertical  section  that 
Count  Spec  made  through  the  primitive  mouth  or  streak  of  a 
very  young  human  germinal  disk  (Fig.  100)  clearly  shows 
that    here    again    the    four    secondary   germ-layers    are    only 


mv    nip  pr      ul 


/: 


ink 


ik      -f^ 

Fig.  mo— Transverse  section  of  the  primitive  groove  (or  primitive 
mouth)  Of  a  hare.  (  From  Van  Beneden. )  pr  primitive  mouth,  id  lips  of  same 
(primitive  lip-.),  at  and  ik  outer  and  inner  germinal  layers,  mk  middle  germinal 
layer,  mp  parietal  layer,  «;•  visceral  layer  of  the  mesoderm. 

inseparably  connected  at  the  primitive  streak,  and  that  here 
also  the  two  flattened  ccelom-pouches  (mk)  extend  centri- 
fugally  to  right  and  left  from  the  primitive  mouth  between 
the  outer  and  inner  germinal  layers.  In  this  case,  too,  the 
middle  germinal  layer  consists  from  the  first  of  two  separate 
strata  of  cells,  the  parietal  (mp  )  and  visceral  (mv)  mesoblasts. 
These  concordant  results  of  the  best  recent  investigations 
(which  have  been  confirmed  by  the  observations  of  a  number 
of  scientists  I  have  not  enumerated)  prove  the  unity  of  the 
vertebrate-stem  in  point  of  ccelomation,  no  less  than  of 
^astrulation.  In  both  respects  the  invaluable  amphioxus — 
the  sole  living  survivor  ot  the  acrania — is  found  to  be  the 
original  model   that  has  preserved  for  us  in  palingenetic  form 


THE  CCELOM  THEORY 


by  a  tenacious  heredity  these  most  important  embryonic 
processes.  From  this  primary  model  of  construction  we  can 
cenogenetically  deduce  all  the  embryonic  forms  of  the  other 
vertebrates,  the  craniota,  by  secondary  modifications.  My 
thesis  of  the  universal  formation  of  the  gastrula  by  folding  of 
the  blastula  has  now  been  clearly  proved  for  all  the  verte- 
brates ;  so  also  has  been  Hertwig's  thesis  of  the  origin  of  the 
middle  germinal  layers  by  the  folding  of  a  couple  of  ccelom- 
pouches  which  appear  at  the  border  of  the  primitive  mouth. 
Just  as  the  gastraja-theory  explains  the  origin  and  identity  of 
the  two  primary  layers,  so  the  ccelom-theory  explains  those 
of  the  four  secondary  layers.     The  point  of  origin  is  always 


ik  ui     p 


Fig.   i oo.— Transverse  section  of  the  primitive  mouth  (or  groove) 

Of  a  human  embryo  (at  the  coelomula  stage).  (From  Count  Spec.)  pr 
primitive  mouth,  ul  lips  of  same  (primitive  folds),  ttk  and  ik  outer  and  inner 
germinal  layers,  ink  middle  layer,  inp  parietal  layer,  /«;•  visceral  layer  of  the 
mesoblasts. 

the  properistoma,  the  border  of  the  original  primitive  mouth 
of  the  gastrula,  at  which  the  two  primary  layers  pass  directly 
into  each  other. 

Moreover,  the  coelomula  is  important  as  the  immediate 
source  of  the  chordula,  the  ontogenetic  reproduction  of  the 
ancient,  typical,  unarticulated,  vermalia-form,  which  has  an 
axial  chorda  between  the  dorsal  nerve-tube  and  the  ventral 
gut-tube.  This  instructive  chordula  (Figs.  86-89)  provides  a 
valuable  support  of  our  phylogeny ;  it  indicates  the  important 
moment  in  our  stem-history  at  which  the  stem  of  the 
chordoma  (tunicates  and  vertebrates)  parted  for  ever  from  the 
divergent  stems  of  the  other  metazoa  (articulates,  echino- 
derms,  and  molluscs). 


THE  ClEI.OM  THEORY 


I  may  express  here  my  opinion,  in  the  form  of  a  chorckea- 
theory,  that  the  characteristic  chordula-larva  of  the  ehordonia 
has  in  reality  this  great  palingenetic  significance — it  is  the 
typical  reproduction  (preserved  by  heredity)  of  the  ancient 
common  stem-form  of  all  the  vertebrates  and  tunieates,  the 
long-extinct  chordcea.  We  will  return  in  the  twentieth 
Chapter  to  these  worm-like  ancestors  which  stand  out  as 
luminous  points  in  the  obscure  stem-history  of  the  inverte- 
brate ancestors  of  our  race.  (Cf.  also  the  eighth  and  ninth 
Tables,  as  to  the  six  fundamental  organs  and  their  functions  in 
the  chordaja.) 


SEVENTH  TABLE 

SYNOPSIS  OF  THE  NAMES  OF  THE  GERMINAL 
LAYERS 

(synonyms  of  the  four  secondary  layers) 


I.  Eetoderma. 

II.  Mesoderma. 

III.  Entoderma. 

Outer  layer. 

Middle  laver. 

Inner  layer. 

Epiblast. 

Mesoblast. 

Hypoblast. 

Eetoblastus. 

Mesoblastus. 

Endoblastus. 

Sensory  layer 

Motor-germinative   layer 

Trophic  layer 

(sensation). 

(movement  and  reproduction). 

(nutrition). 

Eetoblast. 

Mesoblast  and  Mesenehym. 

Endoblast. 

Sense-layer. 

Muscle-layer. 

Vascular  layer. 

Mucous  layer. 

Neural  layer. 

Parietal  layer. 

Visceral  layer. 

Enteral  layer. 

Outer  limiting: 

Outer  middle 

Inner  middle 

Inner  limiting 

laver. 

layer. 

laver. 

layer. 

Methorium    exter- 

Fibrosum exter- 

Fibrosum inter- 

Methorium inter- 

num. 

num. 

num. 

num. 

Animal 

Animal 

Vegetal 

Vegetal 

covering  layer. 

fibrous  layer. 

fibrous  layer. 

covering  layer. 

Neuroblast. 

Myoblast. 

Gonoblast. 

Enteroblast. 

Lamina 

Lamina 

Lamina 

Lamina 

neurodermalis. 

inodermalis. 

inogas/raiis. 

endogastralis. 

Skin-seuse-layer. 

Skin-fibre-layer. 

Gut-fibre-layer. 

Gut-gland-layer. 

(Chief  products  : 

(Chief  products  : 

(Chief  products  : 

(Chief  products  : 

sense -cells     and 

muscle-cells  and 

sex-cells  and 

gland-cells  and 

nerves : 

skeleton : 

vascular  skin. ) 

gut  epithelium  : 

outer  skin.) 

corium. ) 

mucous  lining.) 

Skin-layer.                Muscle-layer. 
Kpiderm  is.                  Myoderm  is. 

Vascular  layer. 
Haemodermis. 

Mucous  Layer. 
Gastroderm  is. 

Body-wall. 

Somatopleura. 
Animal  double-layer. 

Gut- 
Splanch 

Vegetal  dt 

wall. 

lopleura. 
ubie-layer. 

EIGHTH  TABLE 

SYNOPSIS  OF  THE  ORIGIN  AND  FUNCTION  OF 

THE  SIX   FUNDAMENTAL  ORGANS  OF  THE 

CHORDULA  (—  PHYLETICALLY :  CHORD^EA) 

N.B.  The  eighth  and  the  ninth  Tables  are  for  the  purpose  of  explaining 
my  chordaea-theory,  and  giving  a  clear  general  view  of  the  original  anatomic 
and  physiological  properties  of  the  chordaa,  and  also  of  the  palingenetic 
relation  of  this  ancient  pre-Silurian  stem-form  to  the  corresponding  structures 
in  the  human  embryo. 


Primary  condition 

of  the 
Primitive  Organs. 

Blastophylls. 
Germinal  layers. 


I.  Ectoderm 
(epiblast). 
Outer  layer. 


Secondary  condi- 
tion of  the 
Primitive  Organs. 

Blastoplates. 

Germinal  plates. 


i.  Cerablast 
Horn-layer 
(protective 

layer). 

2.  Neuroblast 
Nerve-layer 
(sensitive 

layer  I. 


Six  Primitive 

Organs  of  the 

ChordEea. 


Morphological 
Primitive  organs. 


Six  Primitive 

Functions  of  the 

I  iiordsa. 


Physiological 

Primitive  functions. 


ft.  Epidermis 

Outer  skin  and    | 

I  its    append- 

1 1         ages.  1 

|  -\  Medullary  tube  j 
Nervous  sys-  ' 
tern  and  sense- 

'.         epithelia,         J 


Protection. 


Sensation. 


II.  Mesoderm 
{  mesoblast). 

3.  Myoblast 
Muscle-layer 

(motor  layer). 

4-  Gonoblast 

(3.  Muscle-layer 
'       Muscular 

s\  stem. 

,  4.  Sexual  layer 

.-j.  Motion. 

Middle  layer. 

(germinative 

layer). 

(gonades  :  ova- 

1             riesand  sper- 

1           maria). 

,4.  Propagation. 

5.  Chordablast 
Chorda-layer 

j-.s.  Chorda 

Axial    rod     as 

5.  Fulcration 

(fulcrative 

1          central   sup- 

I       (support). 

Ml.  Entoderm 

(hypoblast ). 
Inner  layer. 

layer). 
6.  Enteroblast 
Gut-gland- 
layer 

port. 
1  (>.  Gastrodermis 
Epithelia       of 
the  gut   and 

j 

U.  Nutrition. 

(nutritive 

the    viscera] 

1 

layer  1. 

'.         glands. 

J 

NINTH  TABLE 

SYNOPSIS  OF  THE  SIX  FUNDAMENTAL  ORGAN'S 

(A)  AND  THE  THREE  BODY-CAVITIES  (B)  OF 

THE    CHORDULA,  AND   THEIR    ORIGIN    FROM 

THE  GERMINAL  LAYERS. 

A.  The  Fundamental  Organs  of  the  Chordula. 


The  Two  Separation  of                Six  Products  of  the 

Primary  ,  the  Four  Seeon-  Primitive              Germinal 

Germinal  dary  Germinal  Embryonic              Plates  in 

Layers.  Layers.                  Plates.                    Man. 


I.  Primitive 
Covering. 

Epithelium  of  the 

outer  or  upper 

layer  : 

Ectoderm 

or  ectoblast 

(animal  layer). 

Epiblast. 


.  Outer  skin  of 
the  chordula  ( = 
ectoderm  of  the 
chordsea). 


,    Dorsal  median 
part     of    the 


1 1.  Cera  b  1  a  s  t  | 

horn-plate  Ji.    Outer  skin, 

(covering-ecto-  |      hair,  nails. 

I     blast).  I 

' 2.    Neuroblast  C 

medullary  plate       2.    Brain,  spinal 

(nerve-plate).  -       marrow,  sense- 
Nerve  -e  c  t  o-  cells. 

[      blast.  l_ 


f 

3.    Parietal  meso-    C 

■j  and  4.    The  two 

blast    (outer   1 3.   Muscle-system, 

layers     of    the 

layer    of    the  J      skeletal  system, 

ccelom-pouches 

ccelom-pouches)!        corium. 

(outer and  inner 

muscle-plate. 

plates  1.        The 

1 

II.  Primitive 

lateral  parts  of 

4.  Viscera]  meso-    I 

Gut. 

the  dorsal  wall 

blast      (inner      4.  Sex-glands, 

Epithelium  of  the 

of  the  primitive 

layer      of     the  i     vascular  system, 

inner  or  lower 

gut. 

ccelom-pouches)         heart,  blood. 

germ-layer  : 

k     muscle-plate.         1 

Entoderm 

or  endoblast 

S.   Median  part  of 

/"5.   Chorda b    ast 
1      (chorda     ■  'ate) 

15.     Rudiment     of 
the     chorda     in 

(vegetal  layer) 

the  dorsal  wall 

hypoblast. 

of  the  primitive 

|      (axial     en  do-    |      the   vertebral 

gut. 

[     blast). 

^     column. 

6.   Ventral  wall  of 

,-6.   Enteroblast 

,6.    Epithelium     oi 

the       primitive 

|     (gland-endo-    |     alimentary 

gut. 

"j      blast)  (gut-K      canal,    lungs, 
|      epithelium).          A      liver,  etc. 

B.  Primary  Cavities  in  the  Body  of  the  Chordula. 


I.  Animal 
Cavity. 


II.  Vegetal 
Cavity. 


Wall  formed  of 
ectoderm- 
epithelia. 


Walls  formed  of 
entoderm- 
epilhelia. 


1.    Single 
tube. 


and  2b.  Pair  of  ' 
ccelom-pouches. 


3.   Single 
tube. 


1.   Cavity    oi'    the 
nerve-tube. 

Medullary 
Canal. 

2a  and  2b.    Right 
and    left    body- 
cavity. 
Cceloma. 

3.  Cavity    of    the 
permanent  gut, 

Gastroeoel. 


TENTH  TABLE 

SYNOPSIS  OF  THE   FOUR  CHIEF  GROUPS  OF 

THE  METAZOA  THAT   MAY    BE   DISTINGUISHED 

ACCORDING  TO  THE  NUMBER  OF  GERMINAL 

LAYERS 


Germ-group.        Germ-layers. 


Germ-form. 


Animal-classes. 


I.  One-1  a  y  e  re  d 

animate. 

Monoblastiea 

(without  primitiv 
gut 


HI 


Blastoderma 


Blastula. 
....      ,  Blastsads 

\  esicular  larva 

,    .  ,         .  .  (volvocina, 

(with  embryonic 

,  "  catallacta, 

cavity  or  blasto- 

.  magosphaera) 

coel). 


Gastrula. 

Gastraeads 

II.  Two-layered 

i.  Eetoderma 

Cup-larva  (with 

(cyemaria, 

animals. 

(epiblast). 

primitive  gut- 

olynthus, 

Diploblastica 

-\  Entoderma 

cavity  and  primi- 
tive mouth  : 

hydra. 

(  with  primitive  gut  I. 

(hypoblast). 

progaster  and 

ccelenteria). 

1 

prostoma). 

III.  Three-layered      t.  Eetoderma 


animals. 
Triploblastica 
(with  gut-cavity— 
gastro-canal  mv- 
tem — always  with- 
out ami-.,  without 
bodj  -cavity). 


skin-layer. 

Mesoderma 


Mesomula. 

Large  larva  or 


(in    tin-    shape  embryo  with  mas- 

ofmesenchym)  sive  mesenchym 

middle  layer.  between  the  two 

3.  Entoderma  primary  layers. 
gut-layer. 


Most  of  the 

ccelenteria 

(sponges, 

speda, 

corals, 

ctenophora, 

platodes). 

Lowest  ccelomaria. 


I\'.    Four-layered 
animals. 

Tetrablastica 
(with     gut  -  cavity 
ami     body-cavity  ; 

generally  with  anus 

and  blood-vessels). 


1.  Neural  layer 
skin-sense-laj  er 
neuroblast 
.'.  Parietal  layer 
skin-fibre-layer 
myoblast 
.-,.  Visceral  layer 
gut-fibre-layer 
gonoblast. 
4.  Enteral  layer 

gut-gland-layer 
enteroblast 


Coelomula. 
Pouch-larva  or 

embryo  with  gut- 
cavity  and  body- 
cavity.  Gut-wall 
formed  ol  the  two 

inner  layers 
(visceral  layers). 

Body-wall  of  the 

two    on  1 1 

layers. 


Most  of  the 

ccelomaria  : 

vermalia 

(great  majority), 

mollusca, 

echinoderma, 

articulata 

(annelida, 

ciustacca, 

trach 
tunicata, 

\  ertebi 
(acrania, 
craniota). 


-'45 


CHAPTER  XI. 

THE  VERTEBRATE  CHARACTER  OF  MAN 

The  association  of  comparative  anatomy  and  ontogeny.  Place  of  man  in 
zoological  classification.  The  types  or  steins  of  the  animal  kingdom. 
The  phvlogenetic  relations  of  the  twelve  animal  stems.  Protozoa  and 
metazoa.  Coelenteria  and  ccelomaria.  Unity  of  the  vertebrate  stem, 
including'  man.  Essential  features  of  the  vertebrates.  Amphioxus  and  the 
hypothetical  primitive  vertebrate  (prospondylus).  Division  of  the  simple 
bilateral  body  into  head  and  trunk.  Axial  rod  or  chorda.  The  antimera  or 
symmetrical  halves  of  the  body.  Medullary  tube  or  nerve  tube  (brain  and 
spinal  marrow).  Three  pairs  of  sense-organs  (nose,  eyes,  ears |.  Chorda- 
sheath  (perichorda).  Muscles.  Corium.  Epidermis.  Body-cavity.  Alimen- 
tary canal.  Gill-gut  in  the  head-half  of  the  body  ;  liver-gut  in  the  trunk- 
half.  Gills  and  lungs.  Stomach  and  small  intestine.  Liver.  Blood-vessels 
and  heart.  Pro-kidneys  (pronephridia).  Segmental  sex-organs  (gonades). 
Metamerism  or  articulation  of  the  vertebrates.  Monophyletic  origin  of  the 
vertebrates  and  of  the  mammals.  The  milk  apparatus  in  mammals. 
Redundant  milk  glands  and  nipples.  Hypermastism  and  hyperthelism. 
Gynecomastism  (large  milk-forming  breast-glands  in  the  male  sex). 
Apparent  hermaphrodism. 

We  have  now  secured  a  number  of  firm  standing-places  in 
the  labyrinthine  course  of  our  individual  development  by  our 
study  of  the  important  embryonic  forms  which  we  have 
called  the  cytula,  morula,  blastula,  gastrula,  ccelomula,  and 
chordula.  But  we  have  still  in  front  of  us  the  difficult  task  of 
deriving  the  complicated  frame  of  the  human  body,  with  all 
its  different  parts,  organs,  members,  etc.,  from  the  simple 
form  of  the  chordula.  We  have  previously  considered  the 
origin  of  this  four-layered  embryonic  form  from  the  two- 
layered  gastrula.  The  two  primary  germinal  layers,  which 
form  the  entire  body  of  the  gastrula,  and  the  two  middle 
layers  of  the  ccelomula  that  develop  between  them,  are  the 
four  simple  cell-strata  or  epithelia,  which  alone  go  to  the 
formation  of  the  complex  body  of  man  and  the  higher 
animals.  It  is  so  difficult  to  understand  this  construction 
that  we  will  first  seek  a  companion  who  may  help  us  out  of 
many  difficulties. 

This    helpful    associate    is    the    science   of    comparative 
246 


THE  VERTEBRATE  CHARACTER  OF  MAN 


anatomy.  Its  task  is,  by  comparing  the  fully-developed 
bodily  tonus  in  the  various  groups  of  animals,  to  learn  the 
general  laws  of  organisation,  according  to  which  the  body  is 
constructed  ;  at  the  same  time,  it  has  to  determine  the 
affinities  of  the  various  groups  by  critical  appreciation  of  the 
degrees  oi  difference  between  them.  Formerly,  this  work 
was  conceived  in  a  teleological  sense,  and  it  was  sought  to 
find  traces  o\  the  pre-formed  plan  of  the  Creator  in  the  actual 
purposive  organisation  of  animals.  But  comparative  anatomy 
has  gone  much  deeper  since  the  establishment  o(  the  theory 
of  descent  ;  its  philosophic  aim  now  is  to  explain  the  variety 
of  organic  forms  by  adaptation,  and  their  similarity  by 
heredity.  At  the  same  time,  it  has  to  recognise  in  the  shades  o( 
difference  in  form  the  degree  of  blood-relationship,  and  make 
an  effort  to  construct  the  ancestral  tree  of  the  animal  world. 
In  this  way,  comparative  anatomy  enters  into  the  closest 
relations  with  comparative  ontogeny  on  the  one  hand,  and 
with  the  science  of  classification  on  the  other. 

Now,  when  we  ask  wliat  position  man  occupies  among 
the  other  organisms  according  to  the  latest  teaching  of 
comparative  anatomy  and  classification,  and  how  man's 
place  in  the  zoological  system  is  determined  by  comparison 
of  the  developed  bodily  forms,  we  get  a  very  definite  and 
significant  reply;  and  this  reply  gives  us  extremely  important 
conclusions  that  enable  us  to  understand  the  embryonic 
development  and  its  phvlogenetic  purport.  Since  Cuvier  and 
Baer,  since  the  immense  progress  that  was  effected  in  the 
early  decades  o\~  the  nineteenth' century  by  these  two  great 
zoologists,  the  opinion  has  generally  prevailed  that  the  whole 
animal  kingdom  may  be  distributed  in  a  small  number  of 
great  divisions  or  types.  They  are  called  types  because  a 
certain  typical  or  characteristic  structure  is  constantly  pre- 
served within  each  of  these  large  sections.  Since  we  applied 
the  theory  ot  descent  to  this  doctrine  of  types,  we  have 
learned  that  this  common  type  is  an  outcome  o(  heredity  ;  a 
the  animals  of  one  type  are  blood-relatives,  or  members  of 
one  stem,  and  can  be  traced  to  a  common  ancestral  form. 
Cuvier  and    Baer  set  up  four  of  these  types  :   the  vertebrates, 


248  THE   VERTEBRATE  CHARACTER  OF  MAN 

articulates,  molluscs,  and  radiates.  The  former  three  of 
these  are  still  retained,  and  may  be  conceived  as  natural 
phylogenetic  unities,  as  stems  or  phyla  in  the  sense  of  the 
theory  of  descent.1  It  is  quite  otherwise  with  the  fourth 
type — the  radiata.  These  animals,  little  known  as  yet  at  the 
beginning-  of  the  nineteenth  century,  were  made  to  form  a  sort  of 
lumber-room,  into  which  were  cast  all  the  lower  animals  that 
did  not  belong  to  the  other  three  types.  As  we  obtained  a 
closer  acquaintance  with  them  in  the  course  of  the  last  sixty 
years,  it  was  found  that  we  must  distinguish  among  them 
from  four  to  eight  different  types.  In  this  way  the  total 
number  of  animal  stems  or  phyla  has  been  raised  to  eight  or 
twelve  (cf.  Chapter  XX.). 

These  twelve  stems  of  the  animal  kingdom  are,  however, 
by  no  means  co-ordinate  and  independent  types,  but  have 
definite  relations,  partly  of  subordination,  to  each  other,  and 
a  very  different  phylogenetic  meaning.  Hence  they  must 
not  be  arranged  simply  in  a  row  one  after  the  other,  as  was 
generally  done  until  thirty  years  ago,  and  is  still  done  in 
some  manuals.  We  must  distribute  them  in  three  subor- 
dinate principal  groups  of  very  different  value,  and  arrange 
the  various  stems  phylogenetically  on  the  principles  which  I 
laid  down  in  my  Monograph  on  the  Sponges,  and  developed  in 
the  Study  of  the  Gastrcea  Theory.  We  have  first  to  dis- 
tinguish the  unicellular  animals  (protozoa)  from  the  multi- 
cellular tissue-forming  (metazoa  J.  Only  the  latter  exhibit 
the  important  processes  of  segmentation  and  gastrulation  ; 
and  they  alone  have  a  primitive  gut,  and  form  germinal 
layers  and  tissues. 

The  metazoa,  the  tissue-animals  or  gut-animals,  then  sub- 
divide into  two  main  sections,  according  as  a  body-cavity  is 
or  is  not  developed  between  the  primary  germinal  layers. 
We  may  call  these  the  ccetenteria  and  coelomaria  ;  the  former 

1  According  to  the  early  theory  of  types,  those  of  the  animal  kingdom  are 
parallel  and  completely  independent ;  but  according-  to  my  gastrfea  theory  they 
are  divergent  stems,  connected  at  their  root.  This  view  of  the  affinity  of  the 
lower  and  higher  animal-stems,  which  I  first  advanced  in  1872  (in  the  Mono- 
graph on  the  Sponges ),  is  further  developed  in  my  Systematic  Phytogeny  (1896), 
and  compendiously  stated  in  the  tenth  edition  of  the  History  of  Creation  (1902). 


THE  VERTEBRATE  CHARACTER  OF  MAN 


arc  often  also  called  zoophytes  or  coelenterata,  and  the  latter 
bilaterals.  This  division  is  the  more  important  as  the  ccelen- 
teria  (without  COelom)  have  no  blood  and  blood-vessels,  or  an 
anus.      The  coelomaria  (with  body-cavity)  have  generally  an 

anus,  and  blood  and  blood-vessels.  There  are  tour  stems 
belonging  to  the  ccelentejia :  the  gastrseads  ("primitive-gut 
animals"),  sponges,  cnidaria,  and  platodes.  Of  the  coelomaria 
we  can  distinguish  six  stems:  the  vermalia  at  the  bottom 
represent  the  common  stem-group  (derived  from  the  platodes) 
of  these,  the  other  five  typical  stems  oi  the  coelomaria — 
the  molluscs,  echinoderms,  articulates,  tunicates,  and  verte- 
brates— being  evolved  from  them. 

Man  is,  in  his  whole  structure,  a  true  vertebrate,  and 
developes  from  an  impregnated  ovum  in  just  the  same 
characteristic  way  as  the  other  vertebrates.  There  can  no 
longer  be  the  slightest  doubt  about  this  fundamental  fact, 
nor  ot  the  fact  that  all  the  vertebrates  form  a  natural  phylo- 
genetic  unity,  a  single  stem.  The  whole  of  the  members  of 
this  stem,  from  the  amphioxus  and  the  cyclostoma  to  the  apes 
and  man,  have  the  same  characteristic  disposition,  connection, 
and  development  of  the  central  organs,  and  arise  in  the  same 
way  from  the  common  embryonic  form  of  the  chordula. 
Without  going  into  the  difficult  question  of  the  origin  of 
this  stem,  we  must  emphasise  the  fact  that  the  vertebrate 
stem  has  no  direct  affinity  whatever  to  five  of  the  other  ten 
stems  ;  these  five  isolated  phyla  are  the  sponges,  cnidaria, 
molluscs,  articulates,  and  echinoderms.  On  the  other  hand, 
there  are  important  and,  to  an  extent,  close  phylogenetic 
relations  to  the  other  five  stems — the  protozoa  (through  the 
amoeba?),  the  gastrseads  (through  the  blastula  and  gastrula), 
the  platodes  and  vermalia  (through  the  ccelomula),  and  the 
tunicates  (through  the  chordula). 

How  we  are  to  explain  these  phylogenetic  relations  in  the 
present  state  o(  our  knowledge,  and  what  place  is  assigned 
to  the  vertebrates  in  the  animal  ancestral  tree,  will  be  con- 
sidered later  (Chapter  XX.).  For  the  present  our  task  is  to 
make  plainer  the  vertebrate  character  of  man,  and  especial ly 
to  point  out  the  chief  peculiarities  of  organisation   by  which 


25o  THE   VERTEBRATE  CHARACTER  OF  MAX 

the  vertebrate  stem  is  profoundly  separated  from  the  other 
eleven  stems  of  the  animal  kingdom.  Only  after  these  com- 
parative anatomical  considerations  shall  we  be  in  a  position 
to  attack  the  difficult  question  of  our  embryology.  The 
development  of  even  the  simplest  and  lowest  vertebrate  from 
the  simple  chordula  (Figs.  86-89)  is  so  complicated  and 
difficult  to  follow  that  it  is  necessary  to  understand  the 
organic  features  of  the  fully-formed  vertebrate  in  order  to 
grasp  the  course  of  its  embryonic  evolution.  But  it  is 
equally  necessary  to  confine  our  attention,  in  this  general 
anatomic  characterisation  of  the  vertebrate-body,  to  the 
essential  facts,  and  pass  by  all  the  unessential.  Hence,  in 
giving  you  now  an  ideal  anatomic  description  of  the  chief 
features  of  the  vertebrate  and  its  internal  organisation,  I  omit 
all  the  subordinate  points  and  restrict  myself  to  the  most 
important  characteristics. 

Much,  of  course,  will  seem  to  the  reader  to  be  essential 
that  is  only  of  subordinate  and  secondary  interest,  or  even 
not  essential  at  all,  in  the  light  of  comparative  anatomy  and 
embryology.  For  instance,  the  skull  and  vertebral  column 
and  the  extremities  are  non-essential  in  this  sense.  It  is  true 
that  these  parts  are  very  important  physiologically ;  but  for 
the  morphological  conception  of  the  vertebrate  they  are  not 
essential,  because  they  are  only  found  in  the  higher,  not  the 
lower,  vertebrates.  The  lowest  vertebrates  have  neither 
skull  nor  vertebra?,  and  no  extremities  or  limbs.  Even  the 
human  embryo  passes  through  a  stage  in  which  it  has  no 
skull  or  vertebra;;  the  trunk  is  quite  simple,  and  there  is  yet 
no  trace  of  arms  and  legs.  At  this  stage  of  development  man, 
like  every  other  higher  vertebrate,  is  essentially  similar  to  the 
simplest  vertebrate  form,  which  we  now  find  in  only  one 
living  specimen.  This  one  lowest  vertebrate  that  merits  the 
closest  study — undoubtedly  the  most  interesting  of  all  the 
vertebrates  after  man — is  the  famous  lancelet  or  amphioxus, 
to  which  we  have  already  often  referred  (Plates  XVIII.  and 
XIX.).  As  we  are  going  to  study  it  more  closely  later  on 
(Chapters  XVI.  and  XVII.),  I  will  only  make  one  or  two 
passing  observations  on  it  here. 


THE  VERTEBRATE  CHARACTER  OF  MAN 


The  amphioxus  lives  buried  in  the  sand  of  the  sea,  is 
from  g  7  centimetres  long,  and  has,  when  fully  developed, 
the  shape  o(  a  very  simple,  longish,  lancet-like  leaf;  hence 
its  name  of  the  laneelet.  The  narrow  body  is  compressed  on 
both  sides,  almost  equally  pointed  at  the  fore  and  hind  ends, 
without  any  traee  of  external  appendages  or  articulation  of 
the  body  into  head,  neck,  breast,  abdomen,  etc.  Its  whole 
shape  is  so  simple  that  its  first  discoverer  thought  it  was  a 
naked  snail.  It  was  not  until  much  later — half  a  century 
ago — that  the  tiny  creature  was  studied  more  carefully,  and 
was  found  to  be  a  true  vertebrate.  More  recent  investigations 
have  shown  that  it  is  of  the  greatest  importance  in  connection 
with  the  comparative  anatomy  and  ontogeny  of  the  verte- 
brates, and  therefore  witli  human  phylogeny.  The  amphioxus 
reveals  the  great  secret  of  the  origin  of  the  vertebrates 
from  the  invertebrate  vermalia,  and  in  its  development  and 
structure  connects  directly  with  certain  lower  tunicates,  the 
ascidia. 

When  we  make  a  number  of  sections  of  the  body  of  the 
amphioxus,  firstly  vertical  longitudinal  sections  through  the 
whole  body  from  end  to  end,  and  secondly  transverse 
sections  from  right  to  left,  wc  get  anatomic  pictures  of 
the  utmost  instructiveness  (cf.  Figs.  101-105  and  Plates 
XVIII.  and  XIX.).  In  the  main  they  correspond  to 
the  ideal  which  we  form  with  the  aid  of  comparative 
anatomy  and  ontogeny  ot  the  primitive  type  or  build  of 
the  vertebrate — the  long  extinct  form  to  which  the  whole 
stem  owes  its  origin.  As  we  take  the  phylogenetic  unity 
of  the  vertebrate  stem  to  be  beyond  dispute,  and  assume 
a  common  origin  from  a  primitive  stem-form  for  all  the 
vertebrates,  from  amphioxus  to  man,  wc  are  justified  in 
forming  a  definite  morphological  idea  of  this  primitive 
vertebrate  ( prospondylus  or  vcrlebnva ).  Wc  need  only 
imagine  a  few  slight  and  unessential  changes  in  the  real 
sections  of  the  amphioxus  in  order  to  have  this  ideal  anatomic 
figure  or  diagram  of  the  primitive  vertebrate  form,  as  we  see 
in  bigs.  101-105.  The  amphioxus  departs  so  little  from  this 
primitive   form   that    we    may,  in   a   certain   sense,  describe    il 


THE   VERTEBRATE  CHARACTER  OF  MAX 


as  a  modified  "primitive  vertebrate"1  (cf.  Plates  XVIII.  and 
XIX.  with  Figs.  101-105). 

The  outer  form  of  our  hypothetical  primitive  vertebrate 
was  at  all  events  very  simple,  and  probably  more  or  less 
similar  to  that  of  the  lancelet.  The  bilateral  or  bilateral- 
symmetrical  body  is  stretched  out  lengthways  and  compressed 
at  the  sides  (Figs.  101-103),  oval  in  section  (Figs.  104,  105). 
There  are  no  external  articulation  and  no  external  appen- 
dages, in  the  shape  of  limbs,  legs,  or  fins.  On  the  other  hand, 
the  division  of  the  body  into  two  sections,  head  and  trunk, 
was  probably  clearer  in  prospondylus  than  it  is  in  its  little- 
changed  ancestor,  the  amphioxus.  In  both  animals  the  fore 
or  head-half  of  the  body  contains  different  organs  from  the 
trunk,  and  different  on  the  dorsal  from  on  the  ventral  side. 
As  this  important  division  is  found  even  in  the  ascidia,  the 
remarkable  invertebrate  stem-relatives  of  the  vertebrates,  we 
may  assume  that  it  was  also  found  in  the  prochordonia,  the 
common  ancestors  of  both  stems.  It  is  also  very  pronounced 
in  the  young  larva?  of  the  cyclostoma  (Plate  XIX.,  Fig.  16); 
this  fact  is  particularly  interesting,  as  this  palingenetic  larva- 
form  is  in  other  respects  also  an  important  connecting-link 
between  the  higher  vertebrates  and  the  acrania. 

The  head  of  the  acrania,  or  the  anterior  half  of  the  body 
(both  of  the  real  amphioxus  and  the  ideal  prospondylus), 
contains  the  gill-gut  and  heart  in  the  ventral  section  and  the 
brain  and  sense-organs  in  the  dorsal  section.  The  trunk,  or 
posterior  half  of  the  body,  contains  the  liver-gut  and  sexual- 
glands  in  the  ventral  part,  and  the  spinal  marrow  and  most  of 
the  muscles  in  the  dorsal  part. 

In  the  longitudinal  section  of  the  ideal  vertebrate 
(Fig.  101)  we  have  in   the  middle  of  the  body  a   thin  and 


1  The  ideal  figure  of  the  vertebrate  as  given  in  Figs.  101-105  's  a  hypo- 
thetical scheme  or  diagram,  that  has  been  chiefly  constructed  on  the  lines  of 
the  amphioxus,  but  with  a  certain  attention  to  the  comparative  anatomy  and 
ontogeny  of  the  ascidia  and  appendicularia  on  the  one  hand,  and  of  the 
cyclostoma  and  selachii  on  the  other.  This  diagram  has  no  pretension  what- 
ever to  be  an  "exact  picture,"  but  merely  an  attempt  to  reconstruct  hypo- 
thetieally  the  unknown  and  long  extinct  vertebrate  stem-form,  an  ideal 
"  architypus. " 


THE   VERTEBRATE  CHARACTER  OF  MAX 


nil      «'  MB       If     gh         -v 


((      r      ms 


ka 

kg  is 

h 

Fig.   ioi. 

I 

dvs 

«f        f 

it 

/ 

gh 

g    an  e  nay 

_— - — 

^T\ 

Figs.  101-105.—  The  ideal  primi- 
tive vertebrate  (prospondylus). 
Diagram.  Fig.  101  side-view  (from 
the  left).  Fig.  102  back-view.  Fig.  103 
front  view.     Fig.  104  transverse  section 


Fig.   105. 


through  tin-  head  (to  the  left  through  the  gill-pouches,  to  the  right  through  the 
gill-clefts).  Fig.  105  transverse  section  of  the  trunk  (to  the  right  a  pro-renal 
canal  is  affected),  a  aorta,  o/*anu9,au  eye,  b  side-furrow  (primitive  renal 
process),  e  cceloma  (body-cavity),  1/  small  intestine,  e  parietal  eye  (epiphysis), 
f  fin  border  of  the  -.kin,  g  auditory  vesicle,  gh  brain,  /;  heart,  i  muscular 
cavity  (dorsal  coelom-pouch),  £  gill-gut,  ka  \  gill-folds, 

/liver,  inn  stomach,  md  mouth,  ms  muscles,  na  nose  (smell  pit),  n  renal  canals, 
11  apertures  of  same,  o  outer  skin,  />  gullet,  r  spinal  narrow,  s  sexual  glands 
(gonades),  /  corium,  u  kidney-openings  (pores  ^t  the  lateral  furrow),  v  visceral 
vein  (chief  vein),  x  chorda, y  hypophysis  (urinary  appendage),  -  gulli 
or  gill-groove  (hypobranchial  groove). 


254  THE    VERTEBRATE  CHARACTER  OF  MAN 

flexible,  but  stiff,  cylindrical  rod,  pointed  at  both  ends  (ch). 
It  goes  the  whole  length  through  the  middle  of  the  body,  and 
forms,  as  the  central  skeletal  axis,  the  original  structure  of 
the  later  vertebral  column.  This  is  the  axial  rod,  or  chorda 
dorsal  is,  also  called  chorda  vertcbralis,  vertebral  cord,  axial 
cord,  spinal  cord,  notochorda,  or,  briefly,  chorda.  This  solid, 
but  flexible  and  elastic,  axial  rod  consists  of  a  cartilaginous 
mass  of  cells,  and  forms  the  inner  axial  skeleton  or  central 
frame  of  the  body  ;  it  is  only  found  in  vertebrates  and 
tunicates,  not  in  any  other  animals.  As  the  first  structure  of 
the  spinal  column  it  has  the  same  radical  significance  in  all 
vertebrates,  from  the  amphioxus  to  man.  But  it  is  only  in 
the  amphioxus  and  the  cyclostoma  that  the  axial  rod  retains 
its  simplest  form  throughout  life.  In  man  and  all  the  higher 
vertebrates  it  is  found  only  in  the  earlier  embryonic  period, 
and  is  afterwards  replaced  by  the  articulated  vertebral  column. 
The  axial  rod  or  chorda  is  the  real  solid  chief  axis  of  the 
vertebrate  body,  and  at  the  same  time  corresponds  to  the 
ideal  long-axis,  and  serves  to  direct  us  with  some  confidence 
in  the  orientation  of  the  principal  organs.  We  therefore  take 
the  vertebrate-body  in  its  original,  natural  disposition,  in 
which  the  long-axis  lies  horizontally,  the  dorsal  side  upward 
and  the  ventral  side  downward  (Fig.  101).  When  we  make  a 
vertical  section  through  the  whole  length  of  this  long-axis, 
the  body  divides  into  two  equal  and  symmetrical  halves,  right 
and  left.  In  each  half  we  have  originally  the  same  organs  in 
the  same  disposition  and  connection;  only  their  disposal  in 
relation  to  the  vertical  plane  of  section,  or  median  plane,  is 
exactly  reversed  :  the  left  half  is  the  reflection  of  the  right. 
We  call  the  two  halves  antimera  (opposed-parts).  In  the 
vertical  plane  of  section  that  divides  the  two  halves  the 
sagittal  ("arrow")  axis,  or  "dorsoventral  axis,"  goes  from 
the  back  to  the  belly,  corresponding  to  the  sagittal  seam  of 
the  skull.  But  when  we  make  an  horizontal  longitudinal 
section  through  the  chorda,  the  whole  body  divides  into  a 
dorsal  and  a  ventral  half.  The  line  of  section  that  passes 
through  the  body  from  right  to  left  is  the  transverse,  frontal, 
or  lateral  axis  (cf.  Plates  VI.  and  VII.). 


THE  VERTEBRATE  CHARACTER  OF  MAN  255 

The  two  halves  ol~  the  vertebrate  body  that  are  separated 

by  this  horizontal  transverse  axis  and  by  the  chorda  are  of 
quite  different  characters.  The  dorsal  half  is  mainly  the 
animal  part  of  the  body,  and  contains  the  greater  part  of 
what    are    called    the    animal    organs,    the    nervous    system, 

muscular  system,  osseous  system,  etc. — the  instruments  of 
movement  and  sensation.  The  ventral  half  is  essentially  the 
vegetative  half  o\  the  body,  and  contains  the  greater  part  of 
the  vertebrate's  vegetal  organs,  the  visceral  and  vascular 
systems,  sL-\ual  system,  etc. — the  instruments  of  nutrition 
and  reproduction.  Hence  in  the  construction  of  the  dorsal 
half  it  is  chiefly  the  outer,  and  in  the  construction  of  the 
ventral  half  chiefly  the  inner,  germinal  layer  that  is  engaged. 
Each  o(  the  two  halves  developes  in  the  shape  of  a  tube,  and 
encloses  a  cavity  in  which  another  tube  is  found.  The  dorsal 
hall  contains  the  narrow  spinal-column  cavity  or  vertebral 
canal  above  the  chorda,  in  which  lies  the  tube-shaped  central 
nervous  system,  the  medullary  tube.  The  ventral  half 
contains  the  much  more  spacious  visceral  cavity  or  body- 
cavity  underneath  the  chorda,  in  which  we  find  the  alimentary 
canal  and  all  its  appendages. 

The  medullary  tube,  as  the  central  nervous  system  or 
psychic  organ  of  the  vertebrate  is  called  in  its  first  stage, 
consists,  in  man  and  all  the  higher  vertebrates,  of  two 
different  parts  :  the  large  brain,  contained  in  the  skull,  and 
the  long  spinal  cord  which  stretches  from  there  over  the  whole 
dorsal  part  of  the  trunk  (Plate  VII.,  Figs,  n-16  n).  Even  in 
the  primitive  vertebrate  this  composition  is  plainly  indicated. 
The  fore  half  of  the  body,  which  corresponds  to  the  head, 
encloses  a  knob-shaped  vesicle,  the  brain  (g/i);  this  is  pro- 
longed backwards  into  the  thin  cylindrical  tube  of  the  spinal 
marrow  ( r ).  Hence  we  find  here  this  very  important  psychic 
organ,  which  accomplishes  sensation,  will,  and  thought,  in 
the  vertebrates,  in  its  simplest  form.  The  thick  wall  of  the 
nerve-tube,  which  runs  through  the  long  axis  of  the  body 
immediately  over  the  axial  rod,  encloses  a  narrow  central 
canal  filled  with  fluid  (Figs.  101-105  r).  We  still  find  the 
medullary  tube    in    this   verv  simple    form   for  a  time  in   the 


THE  VERTEBRATE  CHARACTER  OF  MAX 


embryo  of  all  the  vertebrates  (cf.  Plate  VII.,  Figs.  11-13), 
and  it  retains  this  form  in  the  amphioxus  throughout  life  ; 
only  in  the  latter  case  the  cylindrical  medullary  tube  barely 
indicates  the  separation  of  brain  and  spinal  cord.  The 
lancelet's  medullary  tube  runs  nearly  the  whole  length  of 
the  body,  above  the  chorda,  in  the  shape  of  a  long  thin  tube 
of  almost  equal  diameter  throughout  (Plate  XIX.,  Fig.  15), 
and  there  is  only  a  slight  swelling  of  it  right  at  the  front  to 
represent  the  rudiment  of  a  cerebral  lobe.  It  is  probable 
that  this  peculiarity  of  the  amphioxus  is  connected  with  the 
partial  atrophy  of  its  head,  as  the  ascidian  larva?  (Plate  XVIII., 
Fig.  5)  on  the  one  hand  and  the  young  cyclostoma  (Plate  XIX. , 
Fig.  16)  on  the  other  clearly  show  a  division  of  the  vesicular 
brain,  or  head-marrow,  from  the  thinner,  tubular  spinal 
marrow. 

Probably  we  must  trace  to  the  same  phylogenetic  cause 
the  defective  nature  of  the  sense-organs  of  the  amphioxus, 
which  we  will  describe  later  (Chapter  XVI.).  ProspOndylus, 
on  the  other  hand,  has  probably  had  three  pairs  of  sense- 
organs,  though  of  a  simple  character,  a  pair  of,  or  a  single 
olfactory  depression,  right  in  front  (Figs.  101,  102,  11a),  a  pair 
of  eyes  (au)  in  the  lateral  walls  of  the  brain,  and  a  pair  of 
simple  auscultory  vesicles  (g)  behind.  There  was  also, 
perhaps,  a  single  parietal  or  "  pineal  "  eye  at  the  top  of  the 
skull  (epiphysis,  e ). 

In  the  vertical  median  plane  (or  middle  plane,  dividing 
the  bilateral  body  into  right  and  left  halves)  we  have  in  the 
acrania,  underneath  the  chorda,  the  mesentery  and  visceral 
tube,  and  above  it  the  medullary  tube  ;  and  above  the  latter 
a  membranous  partition  of  the  two  halves  or  antimera  of  the 
body.  With  this  partition  is  connected  the  mass  of  connec- 
tive tissue  which  acts  as  a  sheath  both  for  the  medullary  tube 
and  the  underlying  chorda,  and  is,  therefore,  called  the 
chord-sheath  (perichorda) ;  it  originates  from  the  dorsal  and 
median  part  of  the  ccelom-pouches  which  we  shall  call  the 
skeleton  plate  or  "  sclerotom "  in  the  craniote  embryo. 
In  the  latter  the  chief  part  of  the  skeleton — the  vertebral 
column  and   skull — developes  from  this  chord-sheath  ;  in  the 


THE  VERTEBRATE  CHARACTER  OF  MAX  -'57 

acrania  it  retains  its  simple  form  as  a  soft  connective  matter, 
from  which  are  formed  the  membranous  partitions  between 
the  various  muscular  plates  or  myotomes  (Figs.  101,  102,  ms). 

To  the  right  and  left  of  the  cord-sheath,  at  each  side  of  the 
medullary  tube  and  the  underlying  axial  rod,  we  find  in  all 
the  vertebrates  the  large  masses  of  muscle  that  constitute  the 
musculature  of  the  trunk  and  effect  its  movements.  Although 
these  are  very  elaborately  differentiated  and  connected  in  the 
developed  vertebrate  (corresponding  to  the  many  differentiated 
parts  of  the  bony  skeleton),  in  our  ideal  primitive  vertebrate 
we  can  distinguish  only  two  pairs  of  these  principal  muscles, 
which  run  the  whole  length  of  the  body  parallel  to  the 
chorda.  These  are  the  upper  (dorsal)  and  lower  (ventral) 
lateral  muscles  of  the  trunk.  The  upper  (dorsal)  muscles,  or 
the  original  dorsal  muscles  (Fig.  105  ms),  form  the  thick 
mass  of  flesh  on  the  back.  The  lower  (ventral)  muscles,  or 
the  original  muscles  of  the  belly,  form  the  fleshy  wall  of  the 
abdomen.  Both  sets  are  articulated,  and  consist  of  a  double 
row  of  muscular  plates  (Figs.  101,  102  //is);  the  number  of 
these  myotomes  determines  the  number  of  joints  in  the  trunk, 
or  metamera.  The  myotomes  are  also  developed  from  the 
thick  wall  of  the  coelom-pouches  (Fig.  105  /). 

Outside  this  muscular  tube  we  have  the  external  envelope 
of  the  vertebrate  body,  which  is  known  as  the  corium  or 
cutis  (  Plate  VI.  /).  This  strong  and  thick  envelope  consists, 
in  its  deeper  strata,  chiefly  of  fat  and  loose  connective  tissue, 
and  in  its  upper  layers  of  cutaneous  muscles  and  firmer 
connective  tissue.  It  covers  the  whole  surface  of  the  fleshy 
body,  and  is  of  considerable  thickness  in  all  the  craniota. 
But  in  the  acrania  the  corium  is  merely  a  thin  plate  of 
connective  tissue,  an  insignificant  "  corium-plate "  (lamella 
con'/,  Figs.   101-105  I )• 

Immediately  above  the  corium  is  the  outer  skin  (epidermis, 
a),  the  general  covering  of  the  whole  outer  surface.  In  the 
higher  vertebrates  the  hairs,  nails,  feathers,  claws,  scales, 
etc.,  grow  out  of  this  epidermis.  It  consists,  with  all  its 
appendages  and  products,  of  simple  cells,  and  has  no  blood- 
vessels.     Its  cells  are  connected  with   the  terminations  of  the 


THE   VERTEBRATE  CHARACTER  OF  MAN 


sensory  nerves.  Originally,  the  outer  skin  is  a  perfectly 
simple  covering  of  the  outer  surface  of  the  body,  composed 
only  of  homogeneous  cells — a  permanent  horn-plate.  In  this 
simplest  form,  as  one-layered  epithelium,  we  find  it,  at  first, 
in  all  the  vertebrates,  and  throughout  life  in  the  acrania.  It 
afterwards  grows  thicker  in  the  higher  vertebrates,  and 
divides  into  two  strata — an  outer,  firmer  horn-layer  and  an 
inner,  softer  mucus-layer ;  also  a  number  of  external  and 
internal  appendages  grow  out  of  it :  outwardly,  the  hairs, 
nails,  claws,  etc.,  and  internally,  the  sweat-glands,  fat- 
glands,  etc. 

It  is  probable  that  in  our  primitive  vertebrate  the  skin  was 
raised  in  the  middle  line  of  the  body  in  the  shape  of  a  vertical 
fin  border  (fj.  A  similar  border,  going  round  the 
greater  part  of  the  body,  is  found  to-day  in  the  amphioxus 
and  the  cyclostoma  ;  we  also  find  one  in  the  tail  of  fish-larva? 
and  tadpoles. 

Now  that  we  have  considered  the  external  parts  of  the 
vertebrate  and  the  animal  organs,  which  mainly  lie  in  the 
dorsal  half,  above  the  chorda,  we  turn  to  the  vegetal  organs, 
which  lie  for  the  most  part  in  the  ventral  half,  below  the 
axial  rod.  Mere  we  find  a  large  body-cavity  or  visceral 
cavity  in  all  the  craniota.  The  spacious  cavity  that  encloses 
the  greater  part  of  the  viscera  corresponds  to  only  a  part  of 
the  original  cceloma,  which  we  considered  in  the  tenth 
Chapter ;  hence  it  may  be  called  the  metacosloma.  As  a  rule, 
it  is  still  briefly  called  the  cceloma  ;  formerly  it  was  known  in 
anatomy  as  the  pleuroperitoneal  cavity.  In  man  and  the 
other  mammals  (but  only  in  these)  this  cceloma  divides,  when 
fully  developed,  into  two  different  cavities,  which  are 
separated  by  a  transverse  partition — the  muscular  diaphragm. 
The  fore  or  pectoral  cavity  (pleura  cavity)  contains  the 
oesophagus,  heart,  and  lungs  ;  the  hind  or  peritoneal  or 
abdominal  cavity  contains  the  stomach,  small  and  large 
intestines,  liver,  pancreas,  kidneys,  etc.  But  in  the  verte- 
brate embryo,  before  the  diaphragm  is  developed,  the  two 
cavities  form  a  single  continuous  body-cavity,  and  we  find 
it   thus   in   all    the  lower  vertebrates   throughout  life.      This 


THE  VERTEBRATE  CHARACTER  OF  MAN 


body-cavity  is  clothed  with  a  delicate  layer  of  cells,  the  ccelom- 
epithelium.  In  the  acrania  the  ccelom  is  articulated  both 
dorsal  ly  and  ventrally,  as  their  muscular  pouches  and 
primitive  genital  organs  plainly  show  (Fig.   105). 

The  chief  o(  the  viscera  in  the  body-cavity  is  the  alimen- 
tary canal,  the  organ  that  represents  the  whole  body  in  the 
gastrula.  In  all  the  vertebrates  it  is  a  long  tube,  enclosed  in 
the  body-cavity  and  more  or  less  differentiated  in  length,  and 
has  two  apertures — a  mouth  for  taking  in  food  (Figs.  101, 
to,'',  /in/)  and  an  anus  for  the  ejection  of  unusable  matter  or 
excrements  (a/).  With  the  alimentary  canal  (Plates  IV., 
Y.  </)  a  number  of  glands  are  connected  which  are  of  great 
importance  for  the  vertebrate  body,  and  which  all  grow  out  of 
the  canal.  Glands  of  this  kind  are  the  salivary  glands,  the 
lungs,  the  liver,  and  many  smaller  glands.  Nearly  all  these 
glands  are  wanting  in  the  acrania  ;  probablv  there  were  merely 
a  couple  of  simple  hepatic  tubes  (Figs.  101,  103  /)  in  the 
vertebrate  stem-form.  The  wall  of  the  alimentary  canal  and 
all  its  appendages  consists  of  two  different  layers;  the  inner, 
cellular  clothing  is  the  gut-gland-laver,  and  the  outer,  fibrous 
envelope  consists  of  the  gut-fibre-layer;  it  is  mainly  com- 
posed o\  muscular  fibres  which  accomplish  the  digestive 
movements  of  the  canal,  and  of  connective-tissue  fibres  that 
form  a  firm  envelope.  We  have  a  continuation  of  it  in  the 
mesentery,  a  thin,  bandage-like  layer,  by  means  of  which  the 
alimentary  canal  is  fastened  to  the  ventral  side  of  the  chorda, 
originally  the  dorsal  partition  oi  the  two  ccdom-pouches 
(Plate  VI.,  Fig.  8  /).  The  alimentary  canal  is  variously 
modified  in  the  vertebrates  both  as  a  whole  and  in  its  several 
sections,  though  the  original  structure  is  always  the  same, 
and  is  very  simple.  As  a  rule,  it  is  longer  (often  several  times 
longer)  than  the  body,  and  therefore  folded  and  winding  within 
the  body-cavity,  especially  at  the  lower  end.  In  man  and 
the  higher  vertebrates  it  is  divided  into  several  sections, 
often  separated  by  valves— the  mouth,  pharynx,  oesophagus, 
stomach,  small  and  large  intestine,  and  rectum.  All  these 
parts  develop  from  a  very  simple  structure,  which  originally 
(throughout     life     in     the    amphioxus)     runs     from     end     to 


THE  VERTEBRATE  CHARACTER  OF  MAX 


end  under  the  chorda  in  the  shape  of  a  straight  cylindrical 
canal. 

As  the  alimentary  canal  may  be  regarded  morphologically 
as  the  oldest  and  most  important  organ  in  the  body,  it  is 
interesting  to  understand  its  essential  features  in  the  verte- 
brate more  fully,  and  distinguish  them  from  unessential 
features.  In  this  connection  we  must  particularly  note  that 
the  alimentary  canal  of  every  vertebrate  shows  a  very 
characteristic  division  into  two  sections — a  fore  and  a  hind 
chamber.  The  fore  chamber  is  the  head-gut  or  branchial 
gut  (Figs.  101-103,  p,  k),  and  is  chiefly  occupied  with  respira- 
tion. The  hind  section  is  the  trunk-gut  or  hepatic  gut,  which 
accomplishes  digestion  (ma,  d ').  In  all  vertebrates  there  are 
formed,  at  an  early  stage,  to  the  right  and  left  in  the  fore-part 
of  the  head-gut,  certain  special  clefts  that  have  an  intimate 
connection  with  the  original  respiratory  apparatus  of  the 
vertebrate — the  branchial  (gill)  clefts  (ksj.  All  the  lower 
vertebrates,  the  amphioxus,  lampreys,  and  fishes,  are  con- 
stantly taking  in  water  at  the  mouth,  and  letting  it  out  again 
by  the  lateral  clefts  of  the  gullet.  This  water  serves  for 
breathing.  The  oxygen  contained  in  it  is  inspired  by  the 
blood-canals,  which  spread  out  on  the  parts  between  the  gill- 
clefts,  the  gill-arches  (kg).  These  very  characteristic 
branchial  clefts  and  arches  are  found  in  the  embryo  of  man 
and  all  the  higher  vertebrates  at  an  early  stage  of  develop- 
ment, just  as  we  find  them  throughout  life  in  the  lower  verte- 
brates (Plates  VIII. -XIII.).  However,  these  clefts  and 
arches  never  act  as  respiratory  organs  in  the  mammals, 
birds,  and  reptiles,  but  gradually  develop  into  quite  different 
parts.  Still,  the  fact  that  they  are  found  at  first  in  the 
same  form  as  in  the  fishes  is  one  of  the  most  interesting 
proofs  of  the  descent  of  these  three  higher  classes  from  the 
fishes. 

Not  less  interesting  and  important  is  an  organ  that 
developes  from  the  ventral  wall  in  all  vertebrates — the  gill- 
groove  or  hypobranchial-groove.  In  the  acrania  and  the 
ascidia  it  consists  throughout  life  of  a  glandular  ciliated 
groove,   which    runs  clown   from    the    mouth    in    the   ventral 


THE  VERTEBRATE  CHARACTER  OF  MAX  261 

middle  line  of  the  gill-gut,  and  takes  small  particles  of  food 
to  the  stomach  (Fig.  104  2).  But  in  the  c  ran  iota  the 
thyroid  gland  (thyreoidea)  is  developed  from  it,  the  gland 

that  lies  in  front  o\  the  larynx,  and  which,  when  pathologi- 
cally enlarged,  forms  goitre  (struma). 

From  the  head-gut  we  get  not  only  the  gills,  the  organs 
o(  water-breathing  in  the  lower  vertebrates,  but  also  the 
lungs,  the  organs  of  atmospheric  breathing  in  the  live  higher 
classes.  In  these  cases  a  vesicular  fold  appears  in  the  gullet 
of  the  embryo  at  an  early  stage,  and  gradually  takes  the 
shape  of  two  spacious  sacs,  which  are  afterwards  rilled  with 
air.  These  sacs  are  the  two  air-breathing  lungs,  which  take 
the  place  of  the  water-breathing  gills.  But  the  vesicular 
invagination,  from  which  the  lungs  arise,  is  merely  the 
familiar  air-filled  vesicle,  which  we  call  the  floating- 
bladder  of  the  fish,  and  which  alters  its  specific  weight  as 
hydrostatic  organ  or  floating  apparatus.  This  structure  is 
not  found  in  the  lowest  vertebrate  classes — the  acrania  and 
cyelostoma. 

The  second  chief  section  of  the  vertebrate-gut,  the  trunk  or 
liver-gilt,  which  accomplishes  digestion,  is  of  very  simple 
construction  in  the  acrania.  It  consists  of  two  different 
chambers.  The  first  chamber,  immediately  behind  the  gill- 
gut,  is  the  expanded  stomach  (ma);  the  second,  narrower 
and  longer  chamber,  is  the  straight  small  intestine  (d)  :  it 
opens  behind  on  the  ventral  side  by  the  anus  (af).  Near 
the  limit  of  the  two  chambers  in  the  visceral  cavity  we  find 
the  liver,  in  the  shape  of  a  simple  tube  or  blind  sac  (I);  in 
the  amphioxus  it  is  single  (Plate  XIX.,  Fig.  15  lb)  ;  in 
the  prospondylus  it  was  probably  double  (Figs.   101,  103  /). 

Closely  related  morphologically  and  physiologically  to 
the  alimentary  canal  is  the  vascular  system  of  the  vertebrate, 
the  chief  sections  of  which  develop  from  the  fibrous  gut-layer. 
It  consists  o(  two  different  but  directly  connected  parts,  the 
system  o\  blood-vessels  and  that  of  lymph-vessels.  In  the 
passages  of  the  one  we  find  red  blood,  and  in  the  other 
colourless  lymph.  To  the  lymphatic  system  belong,  first  of 
all,  the  lymphatic  canals  proper  or  absorbent  veins,  which  are 


262  THE   VERTEBRATE  CHARACTER  OF  MAX 

distributed  among  all  the  organs,  and  absorb  the  used-up 
juices  from  the  tissues,  and  conduct  them  into  the  venous 
blood  ;  but  besides  these  there  are  the  chyle-vessels,  which 
absorb  the  white  chyle  (or  milk-juice),  the  nutritive  fluid  pre- 
pared by  the  alimentary  canal,  and  conduct  this  also  to  the 
blood. 

The  blood-vessel  system  of  the  vertebrate  has  a  very 
elaborate  construction,  but  seems  to  have  had  a  very  simple 
form  in  the  primitive  vertebrate,  as  we  find  it  to-day  per- 
manently in  the  ringed-worms  (for  instance,  rain-worms)  and 
the  amphioxus.  We  accordingly  distinguish  first  of  all  as 
essential,  original  parts  of  it  two  large  single  blood-canals, 
which  lie  in  the  fibrous  wall  of  the  gut,  and  run  along  the 
alimentary  canal  in  the  median  plane  of  the  body,  one  above 
and  the  other  underneath  the  canal.  These  principal  canals 
give  out  numerous  branches  to  all  parts  of  the  body,  and  pass 
into  each  other  by  arches  before  and  behind  ;  we  will  call 
them  the  primitive  artery  and  the  primitive  vein.  The  first 
corresponds  to  the  dorsal  vessel,  the  second  to  the  ventral 
vessel,  of  the  worms.  The  primitive  or  principal  artery, 
usually  called  the  aorta  (Fig.  101  a),  lies  above  the  gut  in  the 
middle  line  of  its  dorsal  side,  and  conducts  oxidised  or 
arterial  blood  from  the  gills  to  the  body.  The  primitive  or 
principal  vein  (Fig.  103  7>)  lies  below  the  gut,  in  the  middle 
line  of  its  ventral  side,  and  is  therefore  also  called  the  vena 
subintestinalis ;  it  conducts  carbonised  or  venous  blood  back 
from  the  body  to  the  gills.  At  the  branchial  section  of  the 
gut  in  front  the  two  canals  are  connected  by  a  number  of 
branches,  which  rise  in  arches  between  the  gill-clefts.  These 
"  branchial  vascular  arches  "  (kg)  run  along  the  gill-arches, 
and  have  a  direct  share  in  the  work  of  respiration.  The 
anterior  continuation  of  the  principal  vein  which  runs  on  the 
ventral  wall  of  the  gill-gut,  and  gives  off  these  vascular 
arches  upwards,  is  the  branchial  artery  (ka).  At  the  border 
of  the  two  sections  of  the  ventral  vessel  it  enlarges  into  a 
contractile  spindle-shaped  tube  (Figs.  101,  103  h).  This  is 
the  first  outline  of  the  heart,  which  afterwards  becomes  a  four- 
chambered  pump  in  the  higher  vertebrates  and  man.     There 


THE  VERTEBRATE  CHARACTER  OF  MAN  263 

is  no  heart  in  the  amphioxus,  probably  owing  to  degenera- 
tion. In  prospondylus  the  ventral  gill-heart  probably  had 
the  simple  form  in  which  we  still  find  it  in  the  ascidia  and  the 
embryos  of  the  craniota  (Figs.  101,  10,}  h). 

The  kidneys,  which  act  as  organs  of  excretion  or  urinary 
organs  in  all  vertebrates,  have  a  very  different  and  elaborate 
construction  in  the  sections  of  this  stem;  we  will  consider 
them  further  in  the  twenty-ninth  Chapter.  Here  I  need  only 
mention  that  in  our  hypothetical  primitive  vertebrate  they 
probably  had  the  same  form  as  in  the  actual  amphioxus — the 
fore-kidneys  (protonephra).  These  are  originally  made  up 
of  a  double  row  ot  little  canals,  which  directly  convey  the 
used-up  juices  or  the  urine  out  of  the  body-cavity 
(Fig.  105  //).  The  inner  aperture  of  these  pronephridial  canals 
opens  with  a  vibratory  funnel  into  the  body-cavity  ;  the 
external  aperture  opens  in  lateral  grooves  of  the  epidermis, 
a  couple  of  longitudinal  grooves  in  the  lateral  surface  of  the 
outer  skin  (Fig.  105  b).  The  pronephridial  duct  is  formed 
by  the  closing  of  this  groove  to  the  right  and  left  at  the  sides. 
In  all  the  craniota  it  developes  at  an  early  stage  in  the  horn- 
plate  (Plate  VI.,  Figs.  4  a,  5  u) ;  in  the  amphioxus  it  seems  to 
be  converted  into  a  wide  cavity,  the  atrium,  or  pcribranchial 
space  (Plate  XV I II.,  Fig.  13  c). 

Xext  to  the  kidneys  we  have  the  sexual  organs  of  the 
vertebrate.  In  most  of  the  members  of  this  stem  the  two  are 
joined  together  in  a  unified  urogenital  system  ;  it  is  only  in  a 
few  groups  that  the  urinary  and  sexual  organs  are  separated 
(in  the  amphioxus,  the  cyclostoma,  and  some  sections  of  the 
fish-class).  In  man  and  all  the  higher  vertebrates  the  sexual 
apparatus  is  made  up  of  various  parts,  which  we  will  consider 
in  the  twenty-ninth  Chapter.  But  in  the  two  lowest  classes 
of  our  stem,  the  acrania  and  cyclostoma,  they  consist  merely 
of  simple  sexual  glands  or  gonades,  the  ovaries  of  the  female 
sex  and  the  testicles  (spermatid)  of  the  male  ;  the  former 
provide  the  ova,  the  latter  the  sperm.  In  the  craniota  we 
alwavs  find  only  one  pair  of  gonades;  in  the  amphioxus  several 
pairs,  metamerically  arranged.  They  must  have  had  the 
same  form  in  our  hypothetical  prospondylus  (Figs.  101,  10,-5  •»")• 


264  THE  VERTEBRATE  CHARACTER  OF  MAN 

These  segmental  pairs  of  gonades  are  the  original  ventral 
halves  of  the  ccelom-pouches. 

The  organs  which  we  have  now  enumerated  in  this 
general  survey,  and  of  which  we  have  noted  the  characteristic 
disposition,  are  those  parts  of  the  organism  that  are  found  in 
all  vertebrates  without  exception  in  the  same  relation  to  each 
other,  however  much  they  may  be  modified.  We  have 
chiefly  had  in  view  the  transverse  section  of  the  body 
(Figs.  104,  105),  because  in  this  we  see  most  clearly  the 
distinctive  arrangement  of  them.  But  to  complete  our 
picture  we  must  also  consider  the  articulation  or  metamera- 
formation  of  them,  which  has  yet  been  hardly  noticed,  and 
which  is  seen  best  in  the  longitudinal  section.  In  man  and 
all  the  more  advanced  vertebrates  the  body  is  made  up  of  a 
series  or  chain  of  similar  members,  which  succeed  each  other 
in  the  long  axis  of  the  body — the  segments  or  metamera  of 
the  organism.  In  man  these  homogeneous  parts  number 
thirtv-three  in  the  trunk,  but  they  run  to  several  hundred  in 
many  of  the  vertebrates  (such  as  serpents  or  eels).  As  this 
internal  articulation  or  metamerism  is  mainly  found  in  the 
vertebral  column  and  the  surrounding  muscles,  the  sections 
or  metamera  were  formerly  called  pro-vertebrae.  As  a  fact, 
the  articulation  is  by  no  means  chiefly  determined  and  caused 
by  the  skeleton,  but  by  the  muscular  system  and  the 
segmental  arrangement  of  the  kidneys  and  gonades.  How- 
ever, the  composition  from  these  pro-vertebrae  or  internal 
metamera  is  usually,  and  rightly,  put  forward  as  a  prominent 
character  of  the  vertebrate,  and  the  manifold  division  or 
differentiation  of  them  is  of  great  importance  in  the  various 
groups  of  the  vertebrates.  But  as  far  as  our  present  task — 
the  derivation  of  the  simple  body  of  the  primitive  vertebrate 
from  the  ehordula — is  concerned,  the  articulate  parts  or 
metamera  are  of  secondary  interest,  and  we  need  not  go  into 
them  just  now. 

The  characteristic  composition  of  the  vertebrate  body 
developes  from  the  embryonic  structure  in  the  same  way  in 
man  as  in  all  the  other  vertebrates.  As  all  competent 
experts  now  admit  the  monophyletic  origin  of  the  vertebrates 


THE  I 'ER TEBR.  I  IF  t  II.  1  R.  I ( ' TER  OF  MAX  265 


on  the  strength  o(  this  significant  agreement,  and  this 
"common  descent  of  all  the  vertebrates  from  one  original 
Stem-form  "  is  admitted  as  an  historical  fact,  we  have  found 
the  answer  to  "  the  question  of  all  questions."  We  may, 
moreover,  point  out  that  this  answer  is  just  as  certain  and 
precise  in  the  case  of  the  origin  of  man  from  the  mammals. 
This  advanced  vertebrate  class  is  also  monophyletic,  or  has 
evolved  from  a  common  stem-group  of  lower  vertebrates 
(reptiles,  and,  earlier  still,  amphibia).  This  follows  from  the 
fact  that  the  mammals  are  clearly  distinguished  from  the 
other  classes  of  the  stem,  not  merely  in  one  striking  par- 
ticular, hut  in  a  whole  group  of  distinctive  characters. 

It  is  only  in  the  mammals  that  we  find  the  skin  covered 
with  hair,  the  breast-cavitv  separated  from  the  abdominal 
cavity  by  a  complete  diaphragm,  and  the  larynx  provided 
with  an  epiglottis.  The  mammals  alone  have  three  small 
auscultory  bones  in  the  tympanic  cavity — a  feature  that  is 
connected  with  the  characteristic  modification  of  their 
maxillary  joint.  Their  red  blood-cells  have  no  nucleus, 
whereas  this  is  retained  in  all  other  vertebrates.  Finally,  it 
is  only  in  the  mammals  that  we  find  the  remarkable  function 
of  the  breast-structure  which  has  given  its  name  to  the  whole 
class — the  feeding  of  the  young  by  the  mother's  milk.  The 
mammary  glands  which  serve  this  purpose  are  interesting  in 
so  many  ways  that  we  may  devote  a  few  lines  to  them  here. 

As  is  well  known,  the  lower  mammals,  especially  those 
which  beget  a  number  of  young  at  a  time,  have  several 
mammary  glands  at  the  breast.  Hedge-hogs  and  sows 
have  five  pairs,  mice  four  to  the  pairs,  dogs  and  squirrels 
four  pairs,  cats  and  bears  three  pairs,  most  of  the 
ruminants  and  many  of  the  rodents  two  pairs,  each 
provided  with  a  teat  or  nipple  ( maslos ).  In  the 
various  genera  of  the  half-apes  (lemures)  the  number 
varies  a  good  deal.  On  the  other  hand,  the  bats  and  apes, 
which  only  beget  one  young  at  a  time  as  a  rule,  have  only 
one  pair  of  mammary  glands,  and  these  are  found  at  the 
hrca^t  as  in  man. 

These    variations    in     the    number    or    structure    of    the 


266  THE  VERTEBRATE  CHARACTER  OF  MAX 

mammary  apparatus  ( mammarium )  have  become  doubly 
interesting  in  the  light  of  recent  research  in  comparative 
anatomy.  It  has  been  shown  that  in  man  and  the  apes  we 
often  find  redundant  mammary  glands  (hypermastism)  and 


Fig.  106  A,  B,  C,  D.—  Instances  of  redundant  mammary  glands  and 

nipples^  hypermastism ).  As.  pair  of  small  redundant  breasts  (with  two  nipples 
on  the  left)  above  the  large  normal  ones;  from  a  45-year-old  Berlin  woman, 
who  had  had  children  17  times  (twins  twice).  (From  Hansemann.)  B  the 
highest  number:  ten  nipples  (all  giving  milk),  three  pairs  above,  one  pair 
below,  the  large  normal  breasts  ;  from  a  22-year-old  servant  at  Warschau. 
(From  Netigebaur.)  C  three  pairs  of  nipples  :  two  pairs  on  the  normal  glands 
and  one  pair  above;  from  a  19-year-old  Japanese  maiden.  D  four  pairs  of 
nipples:  one  pair  above  the  normal  and  two  pairs  of  small  accessory  nipples 
underneath;  from  a  22-year-old  Baden  soldier.      (From   Wicderslnim.) 

corresponding  teats  (hyperllielism)  in  both  sexes.  Fig.  106 
shows  four  cases  of  this  kind — A,  B,  and  Cof  three  women, 
and  D  of  a  man.  They  prove  that  all  the  above-mentioned 
numbers    may  be  found   occasionally  in   man.     Fig.  106  A 


THE   VERTEBRATE  CHARACTER  OF  MAN  267 

shows  the  breast  of  a  Berlin  woman  who  had  had  children 
seventeen  times,  and  who  has  a  pair  of  small  accessory 
breasts  (with  two  nipples  on  the  left  one)  above  the  two 
normal  breasts;  this  is  a  common  occurrence,  and  the  small 
soft  pad  abo\e  the  breast  is  not  infrequently  represented  in 
ancient  statues  of  Venus.  In  Fig.  106  C  we  have  the  same 
phenomenon  in  a  Japanese  girl  o\~  nineteen,  who  has  two 
nipples  on  each  breast  besides  (three  pairs  altogether). 
Fig.  106  D  is  a  man  of  twenty-two  with  four  pairs  of 
nipples  (as  in  the  dog),  a  small  pair  above  and  two  small 
pairs  beneath  the  large  normal  teats.  The  maximum 
number  oi  five  pairs  (as  in  the  pig  and  hedge-hog)  was 
found  in  a  Polish  servant  of  twenty-two  who  had  had 
several  children  ;  milk  was  given  by  each  nipple  ;  there  were 
three  pairs  of  redundant  nipples  above  and  one  pair  under- 
neath the  normal  and  very  large  breasts  (Fig.  106  B). 

A  number  of  recent  investigations  (especially  among 
recruits)  have  shown  that  these  things  arc  not  uncommon  in 
the  male  as  well  as  the  female  sex.  They  can  only  be 
explained  by  phylogeny,  which  attributes  them  to  atavism 
and  latent  heredity.  The  earlier  ancestors  of  all  the  primates 
(including  man)  were  lower  placentals,  which  had,  like  the 
hedge-hog  (one  of  the  oldest  forms  of  the  living  placentals), 
several  mammary  glands  (five  or  more  pairs)  in  the  abdominal 
skin.  In  the  apes  and  man  only  a  couple  of  them  are 
normally  developed,  but  from  time  to  time  we  get  a  develop- 
ment of  the  atrophied  structures.  Special  notice  should  be 
taken  of  the  arrangement  of  these  accessory  mammae  ;  they 
form,  as  is  clearly  seen  in  Fig.  106  B  and  /),  two  long 
rows,  which  diverge  forward  (towards  the  arm-pit),  and 
converge  behind  in  the  middle  line  (towards  the  loins).  The 
milk-glands  of  the  polymastic  lower  placentals  are  arranged 
in  similar  lines. 

The  phylogenetie  explanation  of  polymastism,  as  given 
in  comparative  anatomy,  has  lately  found  considerable  support 
in  ontogeny.  Hans  Strahl,  E.  Schmitt,  and  others,  have 
found  that  there  are  always  in  the  human  embryo  at  the 
sixth    week  (when   it  is    15    mm.  long)  the  microscopic  traces 


Fig.  107.— A  Greek  gyneeomast. 


77//;  VERTEBRATE  CHARACTER  OF  MAN  269 


of  five  pairs  of  mammary  glands,  and  that  they  are  arranged 
at  regular  distances  in  two  lateral  and  divergent  lines,  which 
Correspond  to  the  mammary  lines.  Only  one  pair  of  them — 
the  central  pair — are  normally  developed,  the  others  atro- 
phying. Hence  there  is  for  a  time  in  the  human  embryo  a 
normal  hyperthelism,  and  this  can  only  be  explained  by  the 
descent  of  man  from  polvthelic  lower  primates  (lemures). 

But  the  milk-gland  of  the  mammal  has  a  great  morpho- 
logical interest  from  another  point  of  view.  This  organ  for 
feeding  the  young  in  man  and  the  higher  mammals  is,  as  is 
known,  found  in  both  sexes.  However,  it  is  usually  active 
only  in  the  female  sex,  and  yields  the  valuable  "  mother's 
milk";  in  the  male  sex  it  is  small  and  inactive,  a  real  rudi- 
mentary organ  of  no  physiological  interest.  Nevertheless, 
in  certain  cases  we  find  the  breast  as  fully  developed  in  man 
as  in  woman,  and  it  may  give  milk  for  feeding  the  young. 

We  have  a  striking  instance  of  this  gynecomastism  (large 
milk-giving  breasts  in  a  male)  in  Fig.  107.  I  owe  the 
photograph  (taken  from  life)  to  the  kindness  of  Dr.  Ornstein, 
of  Athens,  a  German  physician,  who  has  rendered  service  by 
a  number  of  anthropological  observations  (for  instance,  in 
several  cases  of  tailed  men).  The  gynecomast  in  question  is 
a  Greek  recruit  in  his  twentieth  vear,  who  has  both  normally 
developed  male  organs  and  a  very  pronounced  female  breast. 
It  is  noteworthy  that  the  other  features  of  the  structure  are 
in  accord  with  the  softer  forms  of  the  female  sex.  It  reminds 
us  of  the  marble  statues  of  hermaphrodites  which  the  ancient 
Greek  and  Roman  sculptors  often  produced.  Hut  the  man 
would  only  be  a  real  hermaphrodite  if  he  had  ovaries  internally 
besides  the  (externally  visible)  testicles. 

I  observed  a  very  similar  case  during  my  stay  in  Ceylon 
(at  Belligemnia)  in  188 1.  A  young  Cinghalese  in  his  twenty- 
fifth  year  was  brought  to  me  as  a  curious  hermaphrodite,  half- 
man  and  half-woman.  His  large  breasts  gave  plenty  o( 
milk  ;  he  was  employed  as  "  male  nurse  "  to  suckle  a  new- 
born infant  whose  mother  had  died  at  birth.  The  outline  of 
his  body  was  softer  and  more  feminine  than  in  the  Greek 
shown    in    Fig.   107.      As   the  Cinghalese   are  small  o(  stature 


THE   VERTEBRATE  CHARACTER  OF  MAN 


and  of  graceful  build,  and  as  the  men  often  resemble  the  women 
in  clothing  (upper  part  of  the  body  naked,  female  dress  on 
the  lower  part)  and  the  dressing  of  the  hair  (with  a  comb), 
I  first  took  the  beardless  youth  to  be  a  woman.  The  illusion 
was  greater,  as  in  this  remarkable  case  gynecomastism  was 
associated  with  cryptorchism — that  is  to  say,  the  testicles  had 
kept  to  their  original  place  in  the  visceral  cavity,  and  had 
not  travelled  in  the  normal  way  down  into  the  scrotum. 
(Cf.  Chapter  XXIX.)  Hence  the  latter  was  very  small,  soft, 
and  empty.  Moreover,  one  could  feel  nothing  of  the  testicles 
in  the  inguinal  canal.  On  the  other  hand,  the  male  organ 
was  very  small,  but  normally  developed  (as  in  Fig.  107).  It 
was  clear  that  this  apparent  hermaphrodite  also  was  a  real 
male. 

Another  case  of  practical  gynecomastism  has  been 
described  by  Alexander  von  Humboldt.  In  a  South 
American  forest  he  found  a  solitary  settler  whose  wife  had 
■died  in  child-birth.  The  man  had  laid  the  new-born  child  on 
his  own  breast  in  despair  ;  and  the  continuous  stimulus  of 
the  child's  sucking  movements  had  revived  the  activity  of  the 
mammary  glands.  It  is  possible  that  nervous  suggestion 
had  some  share  in  it.  Similar  cases  have  been  often  observed 
in  recent  years,  even  among  other  male  mammals  (such  as 
sheep  and  goats). 

The  great  scientific  interest  of  these  facts  is  in  their 
bearing  on  the  question  of  heredity.  The  stem-history  of  the 
mammarium  rests  partly  on  its  embryology  (Chapter  XXIV.) 
and  partly  on  the  facts  of  comparative  anatomy  and 
physiology.  As  in  the  lower  and  higher  mammals  (the 
monotremes,  and  most  of  the  marsupials)  the  whole 
lactiferous  apparatus  is  only  found  in  the  female  ;  and  as 
there  are  traces  of  it  in  the  male  only  in  a  few  younger 
marsupials,  there  can  be  no  doubt  that  these  important  organs 
were  originally  found  only  in  the  female  mammal,  and  that 
ihey  were  acquired  by  these  through  a  special  adaptation  to 
habits  of  life. 

Later,  these  female  organs  were  communicated  to  both 
sexes  by  heredity  ;    and  they  have  been    maintained   in   all 


THE  VERTEBRATE  CHARACTER  OF  MAN 


persons  o(  either  sex,  although  they  are  not  physiologically 

active  in  the  males.  This  normal  permanence  of  the  female 
lactiferous  organs  in  both  sexes  of  the  higher  mammals 
and  man  is  independent  of  any  selection,  and  is  a  fine 
instance  of  the  much-disputed  "  inheritance  of  acquired 
characters." 


ELEVENTH  TABLE 

SYNOPSIS  OF  THE  CHIEF  ORGANS  OF  THE 

PROVERTEBRATES  (THE  HYPOTHETICAL 

PRIMITIVE  VERTEBRATES)  AND  THEIR 

DEVELOPMENT  (PROSPONDYLUS) 


Four  Secondary 

Germinal 

Layers. 


Synonyms  of 
the  Layers. 


Fundamental  Organs  of  the 
Primitive  Vertebrates. 


I.  Sensory  layer 

(skin-sense-layer) 
neuroblast. 

Lamina  neuralis 

outer  limiting' 
layer. 

(Sensation.  J 


Skin-layer 

(Baer). 

Primary  animal 

layer. 


Outer  skin  (epidermis) 

(simple    cell-layer   on   the   outer 
surface  of  the  bodv). 

Nervous  system  (sensorium). 

2.   A.    Medullary    tube     (nervous 
centre). 

2.  B.  Peripheral  nervous  system. 

Sense-organs  (sensillaj. 

3.  A.   Nose  (olfactory  pits). 
3.   B.  Eyes. 

3.   C.   Auscultory  vesicles  (stato- 
cysts). 


II. 


(skii 


Muscular 
layer 

i-fibrous-layer) 
mvoblast. 

Lamina 
parietalis 

outer  middle  layer. 
(Movement.) 


Fleshv-layer 
(Baer). 

(Mainly  used 

for  construction 

of  the  episomites 

and  somato- 

pleura. ) 


4.  Corium 

(cutis-plate). 

5.  Muscular  wall  of  the  trunk 
(motorium) 

(metamerous  lateral  muscles). 

6.  Chord-sheath  (perichorda) 

(skeletal  base). 


III.  Sexual  layer 

(gut-fibrous-layer) 
gonoblast. 

Lamina 
viseeralis 

inner  middle  layer. 
(Reproduction . ) 


Vascular  layer 
(Baer). 

(Mainly  used 
for  construction 
of  the  hypo- 
somites  and 
the  splanchno- 
pleura. ) 


7.  Fore  kidneys  (pronephridia) 

(metamerous  ccelom-canals). 

8.  Sexual  glands  (gonades) 

(metamerous      ventral      coelom- 
pouehes). 

9.  Vascular  system  (vasorium). 

9.   A.   Ventral      principal      vein. 

Heart. 
9.   B.   Dorsal     aorta     (principal 

artery). 

10.  Ventral  muscular  wall  and 
mesentery 

(fibrous  wall  of  the  gut). 
10.  A.  Skeleton  and  muscles  of  the 
gill-arches  (visceral  skeleton ). 
10.  B.  Muscular  wall  of  the  hepatic 


IV.  Glandular 
layer 

(gut-gland-la  ver) 

enteroblast. 

Lamina 

enteralis 

inner  limiting' 

layer. 
(Nutrition. ) 


M  ucous  laver 

(Baer). 

Primary  vegetal 

layer. 


1 1 .  Chorda  dorsalis  (notochorda) 

(axial  rod),  unarticulated. 

12.  Gut-epithelium   (gastro- 
dermis). 

12.   A.    Epithelium  of  the  head  or 

gill-gut. 
12.    B.    Epithelium    of  the    trunk 

or  liver-gut. 


CHAPTER    XII. 

EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA 

Cenogenetic  characteristics  of  amniote  embryology.  The  classic  hen's  egg 
as  .1  source  of  error.  False  antithesis  of  germ  and  yelk.  The  yolk  belongs 
to  the  vegetal  half.  Yelk-germ  and  yelk-glajids  of  the  amphibia.  Flal 
germinal  disk  o','  the  birds  and  reptiles.  Severance  of  it  from  the  yelk-sac. 
Primary,  secondary,  and  tertiary  embryonic  si  tges  of  the  vertebrate.  The 
so-called  blastula  of  the  mammal  (germinal  gut-vesicle  or  blastocyst). 
Its  origin  by  modification  of  the  feeding  of  the  young.  Descent  of  the 
viviparous  mammals  from  oviparous.  Envelopes  of  their  epigastrula 
(covering  layer).  Conversion  of  the  two-layered  into  the  four-layered 
germinal  disk.  Dark  and  tight  germinative  area.  Embryonic  shield 
( embryaspis )  or  dorsal  shield  (  notaspis),  embryonic  formation.  Relation  of 
the  germinative  area  to  the  permanent  gut  (menosoma).  The  continued 
inheritance  and  subsequent  loss  of  the  food-yelk  in  the  vertebrates. 
Influence  of  these  cenogenetic  processes  on  the  modification  of  the  gastrula. 

THE  three  higher  classes  of  vertebrates  which  we  call  the 
amniotes — the  mammals,  birds,  and  reptiles — are  notably 
distinguished  by  a  number  of  peculiarities  of  their  develop- 
ment from  the  five  lower  classes  of  the  stem — the  animals 
without  an  amnion  (anamnia  or  ichthyopoda).  All  the 
amniotes  have  a  distinctive  embryonic  membrane  known  as 
the  amnion  (or  "  water-membrane  "),  and  a  special  embryonic 
appendage — the  allantois.  They  have,  further,  a  consider- 
able yelk-sac,  which  is  filled  with  food-yelk  in  the  reptiles  and 
birds,  and  with  a  clear  corresponding  fluid  in  the  mammals. 
In  consequence  of  these  cenogenetic  structures,  the  original 
features  of  the  development  of  the  amniotes  are  so  much 
altered  that  it  is  very  difficult  to  reduce  them  to  the  pa j in- 
genetic  embryonic  processes  of  the  lower  amnion-less  verte- 
brates. The  gastraea  theory  shows  us  how  to  do  this,  by 
representing  the  embryology  of  the  lowest  vertebrate,  the 
skull-less  amphioxus,  as  the  original  form,  and  deducing 
from  it,  through  a  series  oi  gradual  modifications,  the  gastru- 
lation  and  ceelomation  of  the  craniota. 

It  was  somewhat  fatal  to  the  true  conception  of  the   chief 
-7.1  T 


EMBRYOXIC  SHIELD  AND  GERMIXATIVE  AREA 


embryonic  processes  of  the  vertebrate  that  all  the  older 
embryologists,  from  Malpighi  (1687)  and  Wolff  (1750)  to 
Baer  (182S)  and  Remak  (1S50),  always  started  from  the 
investigation  of  the  hen's  egg,  and  transferred  to  man  and 
the  other  vertebrates  the  impressions  they  gathered  from 
this.  This  classical  object  of  embryological  research  is,  as 
we  have  seen,  a  source  of  dangerous  errors.  The  large 
globular  food-yelk  of  the  bird's  egg  causes,  in  the  first  place, 
a  flat  discoid  expansion  of  the  small  gastrula,  and  then  so 
distinctive  a  development  of  this  thin  round  embryonic  disk 
that  the  controversy  as  to  its  significance  occupies  a  large 
part  of  embryological  literature. 

One  of  the  most  unfortunate  errors  that  this  led  to  was 
the  idea  of  an  original  antithesis  of  germ  and  yelk.  The 
latter  was  regarded  as  a  foreign  body,  extrinsic  to  the  real 
germ,  whereas  it  is  really  a  part  of  it,  an  embryonic  organ  of 
nutrition.  Many  authors  said  there  was  no  trace  of  the 
embryo  until  a  later  stage,  and  outside  the  yelk  ;  sometimes 
the  two-layered  embryonic  disk  itself,  at  other  times  only 
the  central  axial  portion  of  it  (as  distinguished  from  the 
germinative  area  which  we  will  describe  presently)  was 
taken  to  be  the  first  outline  of  the  embryo.  In  the  light  of 
the  gastraea  theory  it  is  hardly  necessary  to  dwell  on  the 
defects  of  this  earlier  view  and  the  erroneous  conclusions 
drawn  from  it.  In  reality,  the  first  segmentation-cell,  and 
even  the  stem-cell  itself  and  all  that  issues  therefrom,  belong 
to  the  embryo.  As  the  large  original  yelk-mass  in  the 
undivided  egg  of  the  bird  only  represents  an  inclosure  in  the 
greatly  enlarged  ovum,  so  the  later  content  of  its  embryonic 
yelk-sac  (whether  yet  segmented  or  not)  is  only  a  part  of  the 
entoderm  which  forms  the  primitive  gut.  This  is  clearly 
shown  by  the  amphiblastic  ova  of  the  amphibia  and  cyclos- 
toma,  which  explain  the  transition  from  the  archiblastic 
yelk-less  ova  of  the  amphioxus  to  the  large  yelk-filled  ova 
of  the  reptiles  and  birds. 

It  is  precisely  in  the  study  of  these  difficult  features  that 
we  see  the  incalculable  value  of  phylogenetic  considerations 
in   explaining  complex  ontogenetic   facts,   and   the    need   of 


EMBRYONIC  SHIELD  .IX/>  GERMINATIVE  AREA  275 

separating  cenogenetic  phenomena  from  palingenetic.  This 
is  particularly  clear  as  regards  the  comparative  ontogeny  of 
the  vertebrates,  because  here  the  phylogenetic  unity  of  the 
stem  has  been  already  established  by  the  well-known  facts  of 
paleontology  and  comparative  anatomy.  If  this  unity  of  the 
stem,  on  the  basis  of  the  amphioxus,  were  always  borne  in 
mind,  we  should  not  have  these  errors  constantly  recurring. 

A  wrong  idea  of  the  formation  of  the  yelk  not  only  led 
astray  the  most  and  best  of  the  older  embryologists,  but  the 
same  thing  not  infrequently  happens  in  our  time.  We  have 
a  recent  instance  in  the  excellent  work,  On  the  Embryology 
and  Anatomy  of  the  Ceylon  Ichthyophis  Glutinosus.  Those 
admirable  observers,  the  brothers  Paul  and  Fritz  Sarasin, 
formulated  the  thesis,  in  the  third  part  of  this  work  (18S9), 
that  "the  two  germinal  layers  of  the  gastrula  do  not  corre- 
spond to  the  entoderm  and  ectoderm,  but  to  the  blastoderm 
and  yelk  of  the  vertebrate,"  and  thought  they  had  thus 
"  provided  the  foundation  for  a  comparative  embryology  of 
the  animal  kingdom."  On  their  view,  "  the  gastrula  consists 
of  two  layers,  of  which  the  inner  is  the  lecithoblast  and  the 
outer  the  blastoderm." 

The  misinterpretation  oi  facts  and  confusion  of  ideas 
which  lie  at  the  bottom  of  these  opinions  are  due  to  the 
supposition  that  in  every  case  the  yelk  is  a  part  of  the 
vegetal  half  of  the  embryo.  As  the  undivided  food-yelk  is 
only  a  portion  of  the  contents  of  the  vegetal  hemisphere  of 
the  ovum  in  the  unicellular  germ  (the  stem-cell),  so  we  must 
always  regard  the  divided  food-yelk  as  a  part  of  the  ventral 
wall  of  the  primitive  gut  in  the  multicellular  embryo.  The 
yelk  embryo,  or  lecithoblast,  of  Sarasin  is  only  a  limited 
portion  of  the  entoderm — that  portion  which  developes  in  the 
ventral  wall  of  the  primitive  gut  from  its  central  part;  as 
"  yelk-gland  "  ( lec  it  hade  nia )  it  is  just  as  much  a  subordinate 
glandular  part  of  the  whole  gut-tube  as  the  visceral  glands 
(liver,  lungs,  etc.)  that  afterwards  grow  out  of  it.  On  the 
other  hand,  the  dorsal  part  of  the  embryo,  which  Sarasin 
opposes  as  "  blastoderm  "  to  the  ventral  lecithoblast,  is  by  no 
means  the  original  embryonic  membrane  (embracing  all  the 


276  EMBRYONIC  SHIELD  AND  GERM/NATIVE  AREA 


embryonic  cells),   the  real  blastoderm,   but  the  relic  of  the 
entoderm  and  the  whole  of  the  ectoderm. 

In  many  other  cases  also  the  cenogenetic  relation  of  the 
embryo  to  the  food-yelk  has  until  now  given  rise  to  a  quite 
wrong  idea  of  the  first  and  most  important  embryonic  processes 
in  the  higher  vertebrates,  and  has  occasioned  a  number  of  false 
theories  in  the  ontogeny  of  them.  Until  thirty  years  ago  the 
embryology  of  the  higher  vertebrates  always  started  from  the 
position  that  the  first  structure  of  the  embryo  is  a  flat,  leaf- 
shaped  disk  ;  it  was  for  this  reason  that  the  cell-layers  that 
compose  this  germinal  disk  (also  called  germinative  area)  are 
called  "  germinal  layers."  This  flat  germinal  disk  (blasto- 
discusj,  which  is  round  at  first  and  then  oval,  and  which  is 
often  described  as  the  scar  or  cicatricula  in  the  laid  hen's  egg, 
is  found  at  a  certain  part  of  the  surface  of  the  large  globular 
food-yelk.  I  am  convinced  that  it  is  nothing  else  than  the 
discoid,  flattened  gastrula  of  the  birds  (dtscogastrula).  At 
the  beginning  of  germination  the  flat  embryonic  disk  curves 
outwards,  and  separates  on  the  inner  side  from  the  underlying 
large  yelk-ball.  In  this  way  the  flat  layers  are  converted  into 
tubes,  their  edges  folding  and  joining  together  (Fig.  10S). 
As  the  embryo  grows  at  the  expense  of  the  food-yelk,  the 
latter  becomes  smaller  and  smaller ;  it  is  completely 
surrounded  by  the  germinal  layers.  Later  still,  the  remainder 
of  the  food-yelk  only  forms  a  small  round  sac,  the  yelk  sac  or 
umbilical  vesicle  (saccus  vite/linus  or  vestcula  umbilicalis, 
Fig.  108  nb).  This  is  enclosed  by  the  visceral  layer,  is 
connected  by  a  thin  stalk,  the  yelk-duct  (ductus  vitellinus), 
with  the  central  part  of  the  gut-tube,  and  is  finally,  in  most 
of  the  vertebrates,  entirely  absorbed  by  this  ( H ).  The 
point  at  which  this  takes  place,  and  where  the  gut  finally 
closes,  is  the  visceral  navel.  In  the  mammals,  in  which  the 
remainder  of  the  yelk-sac  remains  without  and  atrophies, 
the  yelk-duct  at  length  penetrates  the  outer  ventral  wall.  At 
birth  the  umbilical  cord  proceeds  from  here,  and  the  point  of 
closure  remains  throughout  life  in  the  skin  as  the  navel. 

As  the  older  embryology  of  the  higher  vertebrates  was 
mainly  based   on  the  chick,  and   regarded  the  antithesis  of 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA 


embryo  (or  formative-yelk)  and  food-yelk  (or  yelk-sac)  as 
original,  it  had  also  to  look-  upon  the  flat  leaf-shaped  structure 
oi  the  germinal  disk  as  the  primitive  embryonic  form,  and 
emphasise  the  fact  that  hollow  grooves  were  formed  of  these 
flat  layers  by  folding,  and  closed  tubes  by  the  joining  together 
oi  their  edges. 


Fig.  ioS.  -Severance  of  the  discoid  mammal  embryo  from  the 
yelk-sac,  in  transverse  section  (diagrammatic).  .1  The  germinal  disk 
(li,  hf)  lies  flat  on  one  side  of  the  gill-gut  vesicle  (kb).  />'  In  the  middle  of 
the  germinal  disk  we  find  the  medullary  groove  (tar),  and  underneath  it  the 
chorda  ( ch  ).  C  The  gut-fibre-layer  fdf)  has  been  enclosed  by  the  gut-gland- 
layer  (ad).  l>  The  skin-fibre-layer  (hf)  and  gut-fibre-layer  ft//')  divide  at  the 
periphery  ;  the  gu(  I  1/  )  begins  to  separate  from  the  yelk-sac  or  umbilical  vesicle 
/■'.  The  medullary  tube  ( mr  j  is  closed;  the  body-cavity  (c)  begins  to 
form.  /•'  The  provertebrae  (  w)  begin  to  grow  round  the  medullary  tube  (mr) 
ami  the  chorda  ( rh  ) :  the  gut  (a)  is cul  off  from  the  umbilical  vesiclefni/ 
//  The  vertebrae  I  w)  have  grown  round  the  medullary  tube  (  mr  )  and  chorda  ■, 
the  body-cavity  is  closed,  and  the  umbilical  vesicle  has  disappeared.  The 
amnion  and  serous  membrane  are  omitted. 

The  letters  have  the  same  meaning  throughout  :  /;  horn-plate,  mr  medullary 
tube,  hf  skin-fibre-layer,  tc  provertebrae,  ch  chorda,  c  body-cavity  or  cceloma, 
fibre-layer,  dd  gut-gland-layer,  d  gut-cavity,  nb  umbilical  vesicle. 

This  idea,  which  dominated  the  whole  treatment  o(  the 
embryology  oi  the  higher  vertebrates  until  thirty  years  ago, 
was  totally  false.  The  gastraea  theory,  which  has  its  chief 
application  here,  teaches  us  that  it  is  the  very  reverse  of  the 
truth.  The  cup-shaped  gastrula,  in  the  body-wall  oi  which 
the  two  primary  germinal  lasers  appear  from  the  first  as 
closed    tubes,    is    the    original   embryonic   form    of   all    the 


EMBRY0X1C  SHIELD  AXD  GERMIXATIVE  AREA 


vertebrates,  and  all  the  invertebrate  metazoa  ;  and  the  flat 
germinal  disk  with  its  superficially  expanded  germinal  layers 
is  a  later,  secondary  form,  due  to  the  cenogenetic  formation 
of  the  large  food-yelk  and  the  gradual  spread  of  the  germ- 
layers  over  its  surface.  Hence  the  actual  folding  of  the 
germinal  layers  and  their  conversion  into  tubes  is  not  an 
original  and  primary,  but  a  much  later  and  tertiary,  evolu- 
tionary process.  In  the  phylogeny  of  the  vertebrate 
embryonic  process  we  may  distinguish  the  following  three 
stages  : — 


A.   First  Stage  : 

B.   Second  Stage  : 

C.   Third  Stage  : 

Primary 

Secondary 

Tertiary 

(palingenetic) 

(cenogenetic) 

(cenogenetic) 

embryonic  process. 

embryonic  process. 

embrj'onic  process. 

The  germinal  layers 
form  from  the  first  closed 
tubes,  the  one-layered 
blastula  being  converted 
into  the  two  -  layered 
gastrula  b}'  invagination. 

No  food-yelk. 

(A  mph  ioxus. ) 


The  germinal  layers 
spread  out  leaf-wise, 
food-yelk  gathering  in 
the  ventral  entoderm, 
and  a  large  yelk-sac 
being  formed  from  the 
middle  of  the  gut-tube, 
(Amphibia.) 


The  germinal  layers 
form  a  flat  germinal  disk, 
the  borders  of  which  join 
together  and  form  closed 
tubes,    separating     from 

the  central  yelk-sac. 

( Amniotes.  J 


As  this  theory,  a  logical  conclusion  from  the  gastraea 
theory,  has  been  fully  substantiated  by  the  comparative 
study  of  gastrulation  in  the  last  few  decades,  we  must  exactly 
reverse  the  hitherto  prevalent  mode  of  treatment.  The  yelk- 
sac  is  not  to  be  treated,  as  was  done  formerly,  as  if  it  were 
originally  antithetic  to  the  embryo,  but  as  an  essential  part 
of  it,  a  part  of  its  visceral  tube.  The  primitive  gut  of  the 
gastrula  has,  on  this  view,  been  divided  into  two  parts  in  the 
higher  animals  as  a  result  of  the  cenogenetic  formation  of 
the  food-yelk — the  permanent  or  after-gut  (metagaster)^  or 
the  permanent  alimentary  canal,  and  the  yelk-sac  ( lecithoma ) 
or  umbilical  vesicle.  This  is  very  clearly  shown  by  the  com- 
parative ontogeny  of  the  fishes  and  amphibia.  In  these 
cases  the  whole  yelk  undergoes  cleavage  at  first,  and  forms 
a  yelk-gland,  composed  of  yelk-cells,   in  the  ventral  wall  of 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA 


the  primitive  gut.  Bui  it  afterwards  becomes  so  large  that 
a  part  of  the  yelk  does  not  divide,  and  is  used  up  in  the  yelk- 
sac  that  is  cut  off  outside. 

When  we  make  a  comparative  study  of  the  embryology  of 
the  amphioxus,  the  frog,  the  chick,  and  the  hare  (Plates  II., 
III.),  there  cannot,  in  my  opinion,  be  any  further  doubt  as 
to  the  truth  of  this  position,  which  I  have  held  for  thirty 
years.  Hence  in  the  light  of  the  gastraea  theory  we  must 
regard  the  features  of  the  amphioxus  as  the  only  and  real 
primitive  structure,  departing  very  little  from  the  palingenetic 
embrvonic  form,  among  all  the  vertebrates.  In  the  cyclostoma 
and  the  frog  these  features  are,  on  the  whole,  not  much 
altered  cenogeneticallv,  but  very  much  so  in  the  chick,  and 
most  of  all  in  the  hare.  In  the  bell-gastrula  of  the  amphioxus 
and  in  the  crested  gastrula  of  the  petromyzoa  and  the  frog  the 
germinal  layers  are  found  to  be  closed  tubes  or  vesicles  from 
the  first  (Plate  II.,  Figs.  6,  11).  On  the  other  hand,  the 
chick-embrvo  (in  the  new  laid,  but  not  yet  hatched,  egg)  is  a 
flat  circular  disk,  and  it  was  not  easy  to  recognise  this  as  a 
real  gastrula.  Rauber  and  Goette  have,  however,  achieved 
this.  As  the  discoid  gastrula  grows  round  the  large  globular 
yelk,  and  the  after-gut  or  permanent  gut  then  separates  from 
the  outlying  velk-sac,  we  find  all  the  processes  which  we 
have  shown  (diagrammatically)  in  Fig.  108 — processes  that 
were  hitherto  regarded  as  principal  acts,  whereas  they  are 
merely  secondary. 

The  oldest,  oviparous  mammals,  the  discoblastic  mono- 
tremes,  behave  in  the  same  way  as  the  sauropsida  (reptiles 
and  birds).  But  the  corresponding  embryonic  processes  in 
the  viviparous  mammals,  the  marsupials  and  placentals,  are 
very  elaborate  and  distinctive.  They  were  formerly  quite 
misinterpreted  ;  it  was  not  until  the  publication  of  the  studies 
of  Edward  van  Beneden  (1875)  and  the  later  research  of 
Selenka,  Kuppfer,  Rabl,  and  others,  that  light  was  thrown 
on  them,  and  we  were  in  a  position  to  bring  them  into  line 
with  the  principles  of  the  gastraea  theory  and  trace  them  to  the 
embrvonic  forms  of  the  lower  vertebrates.  Although  there 
is   no    independent   food-yelk,  apart  from  the  formative  yelk, 


2So  EMBRYOXIC  SHIELD  AXD  GERMIXAT1VE  AREA 

in  the  mammal  ovum,  and  although  their  segmentation  is 
total  on  that  account,  nevertheless  a  large  yelk-sac 
( lecithoma )  is  formed  in  their  embryos,  and  the  "  embryo 
proper  "  spreads  leaf-wise  over  its  surface,  as  in  the  reptiles 
and  birds,  which  have  a  large  foqd-yelk  and  partial  segmen- 
tation. In  the  mammals,  as  well  as  in  the  latter,  the  flat, 
leaf-shaped  germinal  disk  (blastodiscus )  separates  from  the 
yelk-sac,  and  its  edges  join  together  and  form  tubes. 

How,  then,  can  we  explain  this  curious  anomaly?  Only 
as  a  result  of  very  characteristic  and  peculiar  cenogenetic 
modifications  of  the  embryonic  process,  the  real  causes  of 
which  must  be  sought  in  the  change  in  the  rearing  of  the 
young  on  the  part  of  the  viviparous  mammals.  These  are 
clearly  connected  with  the  fact  that  the  ancestors  of  the 
viviparous  mammals  were  oviparous  amniotes  like  the 
present  monotremes,  and  only  gradually  became  viviparous. 
This  can  no  longer  be  questioned  now  that  it  has  been 
shown  (1884)  that  the  monotremes,  the  lowest  and  oldest  of 
the  mammals,  still  lav  eggs,  and  that  these  develop  like  the 
discoblastic  ova  of  the  reptiles  and  birds.  Their  nearest 
descendants,  the  marsupials,  formed  the  habit  of  retaining 
the  eggs,  and  developing  them  in  the  oviduct ;  the  latter 
was  thus  converted  into  a  womb  (uterus).  A  nutritive  fluid 
that  was  secreted  from  its  wall,  and  transuded  through  the 
wall  of  the  blastula,  now  served  to  feed  the  embryo,  and  took 
the  place  of  the  food-yelk.  In  this  way  the  original  food- 
yelk  of  the  meroblastic  monotremes  was  gradually  atrophied, 
and  at  last  disappeared  so  completely  that  the  partial  ovum- 
segmentation  of  their  descendants,  the  rest  of  the  mammals, 
once  more  became  total.  From  the  discogastrula  of  the 
former  was  evolved  the  distinctive  epigastrula  of  the  latter. 

It  is  only  by  this  phylogenetic  explanation  that  we  can 
understand  the  formation  and  development  of  the  peculiar, 
and  hitherto  totally  misunderstood,  blastula  of  the  mammal. 
This  vesicular  condition  of  the  mammal  embryo  was  dis- 
covered 200  years  ago  (1677)  by  Regner  de  Graaf.  He 
found  in  the  uterus  of  a  hare  four  days  after  impregnation 
small,    round,    loose,     transparent    vesicles,    with    a    double 


EMBRYONIC  SHIELD  AND  GERM/NATIVE  AREA 


envelope.  However,  Graaf's  discovery  passed  without  recog- 
nition. It  was  not  until  1827  that  these  vesicles  were 
re-discovered  by  Baer,  and  then  more  closely  studied  in 
1S4J  by  Bischoff  in  the  hare  (Figs.  109,  no).  They  are 
found  in  the  womb  of  the  hare,  the  dog,  and  other  small 
mammals,  a  lew  days  after  copulation.  The  mature  ova  o\ 
the  mammal,  when  they  have  left  the  ovary,  are  fertilised 
either  here  or  in  the  oviduct  immediately  afterwards  by  the 
invading  sperm-cells.1  (As  to  the  womb  and  oviduct  see 
Chapter  XXIX.)  The  cleavage  and  formation  of  the 
gastrula  take  place  in  the  oviduct.  Either  here  in  the 
oviduct  or  after  the    mammal   gastrula  has   passed    into  the 


Fig,   109.  Fig.   i  10. 

Fig.  109.    The  visceral  embryonic  vesicle  (blastocysts  or  gastrocystis) 

of  a  hare  11  Ik-  "  blastula"  or  vesicula  blastodermica  of  other  writers).     "  outer 

envelope  (ovolemma),  6  skin-layer  or  ectoderm,  forming  tin'  entire  wall  ol  the 

yelk-vesicle,  c  groups  of  dark  cells,  representing  the  visceral  layer  or  entoderm. 

Fig.   mo.     The  same  in  sections,     Letters  as  above.    </  cavity  of  the 

1  From  Bischoff.  1 

uterus  it  is  converted  into  the  globular  vesicle  which  is 
shown  externally  in  Fig.  109,  and  in  section  in  Fig.  no. 
The  thick,  outer,  structureless  envelope  that  encloses  it  is 
the  original  ovo/emma  or  zona  pellucida,  modified,  and 
clothed  with  a  layer  of  albumin  that  has  been  deposited  on 
the  outside.  From  this  stage  the  envelope  is  called  the 
external   membrane,  the  primarv  chorion  or  prochorion  <  a  1. 


1  In  man  and  the  other  mammals  the  fertilisation  of  the  ova  probably  takes 
place,  as  a  rule,  in  tin'  oviduct  ;  here  the  ova,  which  issue  from  the  Female 
ovary  in  the  shape  ol  the  Graafian  Follicle,  and  enter  tin'  inner  aperture  of  the 
oviduct,  encounter  the  mobile  sperm-cells  of  the  male  seed,  which  pass  into 
the  uterus  at  copulation,  and  from  this  into  the  external  aperture  ol  the 
oviduct.       Impregnation  rarely  takes  place  in  the  ovary  or  In  the  womb. 


EMBRYOXIC  SHIELD  AXD  GERMIXATIVE  AREA 


The  real  wall  of  the  vesicle  enclosed  by  it  consists  of  a 
simple  layer  of  ectodermic  cells  (bj,  which  are  flattened  by 
mutual  pressure,  and  generally  hexagonal  ;  a  light  nucleus 
shines  through  their  fine-grained  protoplasm  (Fig.  m).  At 
one  part  (c)  inside  this  hollow  ball  we  find  a  circular  disc, 
formed  of  darker,  softer,  and  rounder  cells,  the  dark-grained 
entodermic  cells  (Fig.  112). 

The  characteristic  embryonic  form  that  the  developing 
mammal  now  exhibits  has  up  to  the  present  usually  been 
called  the  "blastula"  (Bischoff),  "sac-shaped  embryo"  (Baer), 
"vesicular  embryo"  (vesicula  blastoderm ica,  or,  briefly, 
blastosphcvra ).  The  wall  of  the  hollow  vesicle,  which  consists 
of  a  single  layer  of  cells,  was  called  the  "blastoderm,"  and 

was  supposed  to  be  equi- 
valent to  the  cell-layer  of 
the  same  name  that  forms 
the  wall  of  the  real  blas- 
tula of  the  amphioxus 
(Plate  II.,  Fig.  4)  and 
many  of  the  inverte- 
brates (such  as  monoxe- 
nia,  Fig.  31,  F,  G).  For- 
merly this  real  blastula 
was  generally  believed  to 
be  equivalent  or  homo- 
logous to  the  embryonic  vesicle  of  the  mammal.  However,  this 
is  by  no  means  the  case.  What  is  called  the  "  blastula  "  of  the 
mammal  and  the  real  blastula  of  the  amphioxus  and  many  of 
the  invertebrates  are  totally  different  embryonic  structures. 
The  latter  (blastula)  is  palingenetic,  and  precedes  the  forma- 
tion of  the  gastrula.  The  former  (blastodermic  vesicle)  is 
cenogenetic,  and  follows  gastrulation.  The  globular  wall  of 
the  blastula  is  a  real  blastoderm,  and  consists  of  homogeneous 
(blastodermic)  cells  ;  it  is  not  yet  differentiated  into  the  two 
primary  germinal  layers.  But  the  globular  wall  of  the 
mammal  vesicle  is  the  differentiated  ectoderm,  and  at  one 
point  in  it  we  find  a  circular  disk  of  quite  different  cells — the 
entoderm.     The  round  cavity,  filled  with  fluid,  inside  the  real 


Fig.  11 1. 


Fig.  11 


Fig.  hi.— Four  entodermie  cells  from 

the  embryonic  vesicle  of  the  hare. 

Fig.  112.— Two  entodermie  cells  from 

the  embryonic  vesicle  of  the  hare. 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA 


blastula  is  the  segmentation-cavity.  But  the  similar  cavity 
within  the  mammal  vesicle  is  the  yelk-sac  cavity,  which  is 
connected  with  the  incipient  gut-cavity.  This  primitive  gut- 
cavity  passes  directly  into  the  segmentation-cavity  in  the 
mammals,  in  consequence  of  the  peculiar  cenogenetic  changes 
in  their  gastrulation,  which  we  have  considered  previously 
(cf.  Chapter  IX.). 

For  these  reasons  it  is  very  necessary  to  recognise  the 
secondary  embryonic  vesicle  in  the  mammal  ( gastrocystis  or 
blastocyst  is,  formerly  called  vesicula  blastodermica)  as  a 
characteristic  structure  peculiar  to  this  class,  and  distinguish 
it  carefully  from  the  primary  blastula  of  the  amphioxus  and 
the  invertebrates.  The  wall  of  this  mammal  vesicle  consists 
of  two  different  parts.  The  greater  portion  of  it  is  one- 
layered,  and  formed  only  of  the  ectoderm.  The  smaller  part, 
namely  the  round  disk  that  is  made  up  of  the  two  primary 
germinal  layers,  may  be  called  with  Van  Beneden  the  gastric 
disk  ( gast rodiscus  ,.  The  primary  ectoderm  is  partly  tran- 
sitory (a  temporary  envelope  or  Raub's  "covering  layer"),  and 
is  replaced  by  a  secondary  ectoderm,  which  developes  from  the 
border  of  the  gastric  disk. 

The  small,  circular,  whitish,  and  opaque  spot  which  this 
gastric  disk  forms  at  a  certain  part  of  the  surface  of  the  clear 
and  transparent  embryonic  vesicle  has  long  been  known  to 
science,  and  compared  to  the  germinal  disk  of  the  birds  and 
reptiles.  Sometimes  it  has  been  called  the  germinal  disk 
(discus  blastodermicus),  sometimes  the  germinal  spot  (tache 
embryonnaire),  and  usually  the  germinative  area  (area 
germinattDa).  From  the  area  the  further  development 
of  the  embryo  proceeds.  However,  the  larger  part  of  the 
embrvonic  vesicle  of  the  mammal  is  not  directly  used  tor 
building  up  the  later  body,  but  for  the  construction  o(  the 
temporary  umbilical  vesicle.  The  embryo  separates  from 
this  in  proportion  as  it  grows  at  its  expense ;  the  two 
are  only  connected  bv  the  yelk-duct  (the  stalk  of  the  yelk- 
sac),  and  this  maintains  the  direct  communication  between 
the  cavity  of  the  umbilical  vesicle  and  the  forming  visceral 
cavity  (Fig.   io.S). 


284  EMBRYOXIC  SHIELD  AXD  GERMINATIVE  AREA 

The  germinative  area  or  gastric  disk  of  the  mammal 
consists  at  first  (like  the  germinal  disk  of  birds  and  reptiles) 
merely  of  the  two  primary  germinal  layers,  the  ectoderm  and 
entoderm.  But  soon  there  appears  in  the  middle  of  the 
circular  disk  between  the  two  a  third  stratum  of  cells,  the 
rudiment  of  the  middle  layer  or  fibrous  layer  (mesoderma). 
This  middle  germinal  layer  consists  from  the  first,  as  we  have 
seen  in  the  tenth  Chapter,  of  two  separate  epithelial  plates, 
the  two  lavers  of  the  ccelom-pouches  (parietal  and  visceral). 
However,  in  all  the  amniotes  (on  account  of  the  large  forma- 
tion of  yelk)  these  thin  middle  plates  are  so  firmly  pressed 
together  that  they  seem  to  represent  a  single  layer.  It  is 
thus  peculiar  to  the  amniotes  that  the  middle  of  the  germina- 
tive area  is  composed  of  four  germinal  layers,  the  two 
limiting  (or  primary)  layers  and  the  middle  layers  between 
them  (Figs.  99,  100).  These  four  secondary  germinal  layers 
can  be  clearly  distinguished  as  soon  as  what  is  called  the 
sickle-groove  (or  "embryonic  sickle")  is  seen  at  the  hind 
border  of  the  germinative  area.  At  the  periphery,  however, 
the  germinative  area  of  the  mammal  only  consists  of  two 
layers.  The  rest  of  the  wall  of  the  embryonic  vesicle  consists 
at  first  (but  only  for  a  short  time  in  most  of  the  mammals)  of 
a  single  layer,  the  outer  germinal  layer. 

From  this  stage,  however,  the  whole  wall  of  the  embryonic 
vesicle  becomes  two-layered.  The  middle  of  the  germinative 
area  is  much  thickened  by  the  growth  of  the  cells  of  the 
middle  layers,  and  the  inner  layer  expands  at  the  same  time, 
and  increases  at  the  border  of  the  disk  all  round.  Lying 
close  on  the  outer  layer  throughout,  it  grows  over  its  inner 
surface  at  all  points,  covers  first  the  upper  and  then  the  lower 
hemisphere,  and  at  last  closes  in  the  middle  of  the  inner  layer 
(Figs.  113-117).  The  wall  of  the  embryonic  vesicle  now 
consists  throughout  of  two  layers  of  cells,  the  ectoderm 
without  and  the  entoderm  within.  It  is  only  in  the  centre  of 
the  circular  area,  which  becomes  thicker  and  thicker  through 
the  growth  of  the  middle  layers,  that  it  is  made  up  of  all  four 
layers.  At  the  same  time  small  structureless  tufts  or  warts 
are    deposited    on     the    surface    of    the    outer    ovolemma    or 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA  285 

prochorion,  which    has    been    raised   above    the  embryonic 
vesicle  (Figs.  1  15-1 17  </). 

We  may  now  disregard  both  the  outer  ovolemma  and  the 
greater  part   of  the  vesicle,  and   concentrate  our  attention  on 


Fig. 


Fk 


Fig.  113.  Ovum  of  a  hare  from 
the  iiu-ruN,  tour  mm.  in  diameter.  The 
embryonic  vesicle  ( l> )  lias  withdrawn 
;i  little  from  the  smooth  ovolemma 
In  tlio  middle  ot~  the  ovolemma  we  see 
the  round  germinal  disk  (blast odiscus, 
c),  at  the  edge  of  which  (at  </i  the  inner 
layer  of  the  embryonic  vesicle  is  already 
beginning  to  expand.  (Figs.  113  117 
from  B\ 

Fig.  114.  The  same  ovum,  seen 
in  profile.     Letters  as  in  Fig.  113. 

Fig.  115.  Ovum  of  a  hare  from 
the  uterus,  six  mm.  in  diameter.  The 
blastoberm  is  already  for  the  most  part 
two-layered  (b).  The  ovolemma,  or 
outer  envelope,  is  tufted  (a), 

Fig.  u().     The  same  ovum,  s.-.n  in  profile.     Letters  as  in  Fig.  115. 

Fig.  117.  Ovum  of  a  hare  from  tlu-  uterus,  eight  mm.  in  diameter.  The 
embryonic  vesicle  is  now  nearly  everywhere  two-layered  ( k),  onlj  remaining 
one-layered  below  (at  </). 


Fig. 


2S6  EMBRYOXIC  SHIELD  AXD  GERMIXATIVE  AREA 

the  germinative  area  and  the  four-layered  embryonic  disk.  It 
is  here  alone  that  we  find  the  important  changes  which  lead 
to  the  differentiation  of  the  first  organs.  In  this  it  is 
immaterial  whether  we  examine  the  germinative  area  of  the 
mammal  (of  the  hare,  for  instance)  or  the  germinal  disk  of  a 
bird  or  a  reptile  (such  as  a  lizard  or  tortoise).  The  embryonic 
processes  we  are  now  going  to  consider  are  essentially 
the  same  in  all  members  of  the  three  higher  classes  of  verte- 
brates which  we  call  the  amniotes.  Man  is  found  to  agree  in 
this  respect  with  the  hare,  dog,  ox,  etc. ;  and  in  all  these 
mammals  the  germinative  area  undergoes  essentially  the 
same  changes  as  in  the  birds  and  reptiles.     They  are  most 


Fig.  i  i  8. 
Fig.   i  18.—  Round  germinative  area  of   the  hare,  divided  into  the 

central  light  area  (  area  pellucida)  and  the  peripheral  dark  area  (urea  opaca  ). 
The  light  area  seems  darker  on  account  of  the  dark  ground  appearing 
through  it. 

Fig.    119. — Oval  area,  with   the  opaque  whitish  border  of  the  dark   area 
without. 

frequently  and  accurately  studied  in  the  chick,  because  we 
can  have  incubated  hen's  eggs  in  any  quantity  at  any  stage  of 
development.  Moreover,  the  round  germinal  disk  of  the 
chick  passes  immediately  after  the  beginning  of  incubation 
(within  a  few  hours)  from  the  two-layered  to  the  four-layered 
stage,  the  two-layered  mesoderm  developing  from  the  median 
primitive  groove  between  the  ectoderm  and  entoderm 
(Figs.  85-98). 

The  first  change  in  the  round  germinal  disk  of  the  chick 
is  that  the  cells  at  its  edges  multiply  more  briskly,  and  form 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA  287 

darker  nuclei  in  their  protoplasm.  This  gives  rise  to  a  dark 
ring,  more  or  less  sharply  set  oi'i  from  the  lighter  centre  ol 
the  germinal  disk  (Fig.  1 1 S ) .  From  this  point  the  latter 
takes  the  name  of  the  "light  area"  (area pellucida J,  and  the 
darker  ring  is  called  the  "  dark  area  "  (area  Opaca).  (In  a 
Strong  light,  as  in  Figs.  [18-120,  the  light  area  seems  dark, 
because  the  (.lark  ground  is  seen  through  it  ;  and  the  dark 
area  seems  whiter.)  The  circtilar  shape  of  the  area  now 
changes  into  elliptic,  and  then  immediately  into  oval 
(Figs.  1  10,  120).  One  end  seems  to  be  broader  and  blunter, 
the  other  narrower  and  more  pointed;  the  former  corresponds 
to  the  anterior  and  the  latter  to  the  posterior  section  of  the 
subsequent  body.  At  the  same  time,  we  can  already  trace 
the  characteristic  bilateral  form  of  the  body,  the  antithesis  of 
right  and  left,  hefore  and  behind.  This  will  be  made  clearer 
by  the  ••primitive  streak,"  which  appears  at  the  posterior  end. 
At  an  early  stage  an  opaque  spot  is  seen  in  the  middle  of 
the  clear  germinative  area,  and  this  also  passes  from  a 
circular  to  an  oval  shape.  At  first  this  shield-shaped  marking 
is  very  delicate  and  barely  perceptible  ;  but  it  soon  becomes 
clearer,  and  now  stands  out  as  an  oval  shield,  surrounded  by 
two  rings  or  areas  (Fig.  120).  The  inner  and  brighter  ring 
is  the  remainder  of  the  pellucid  area,  and  the  dark  outer  rin^ 
the  remainder  of  the  opaque  area  ;  the  opaque  shield-like 
spot  itself  is  the  first  rudiment  of  the  dorsal  part  of  the 
embryo.  We  give  it  briefly  the  name  of  embryonic  shield 
(embryaspis) or  dorsal  shield (notaspis). '  Remak  has  called 
it  the  "  double  shield,"  because  it  arises  from  a  shield-shaped 
thickening  of  the  outer  and  middle  germinal  layer.  Inmost 
works  this  embryonic  shield  is  described  as  "the  first 
rudiment  or  trace  of  the  embryo"  or  "primitive  embryo."  But 
this  is  wrong,  though  it  rests  on  the  authority  of  Baer  and, 
Bischoff.  As  a  matter  of  fact,  we  already  have  the  embryo 
in    the   stem-cell,  the  gastrula,  and   all  the  subsequent  stages. 


1  Tin-  germinal  shield  is  at  first  merely  .1  dorsal  shield  in  the  amniotes  ; 
when  tin-  frontal  septum  is  afterwards  formed  between  the  episoma  and 
byposoma,  the  dorsal  shield  appears  as  the  "stem-zone"  in  contrast  to  the 
ventral  body  ("  parietal  zone"  or  yelk-sac). 


EMBRYONIC  SHIELD  AXD  GERMIXATIVE  AREA 


The  embryonic   shield    is    simply  the   first    rudiment   of  the 
dorsal  part,  which  is  the  earliest  to  develop. 

As  the  older  names  of  "  embryonic  rudiment  "  and 
"germinative  area"  are  used  in  many  different  senses — and 
this  has  led  to  a  fatal  confusion  in  ontogenetic  literature — we 
must  explain  very  clearly  the  real  significance  of  these 
important  embryonic  parts  of  the  amniote.  Remak  had 
pointed  out  in  1850  that  it  is  quite  wrong  to  describe  the 
embryonic  shield  or  "  Baer's  shield  "  as  "  the  future  embryo  " 


Fig.  120. 


Fig.  121. 


Fig.  120.—  Oval  germinal  disk  Of  the  hare,  magnified  about  ten  times. 
As  the  delicate,  half-transparent  disk  lies  on  a  black  ground,  the  pellucid  area 
looks  like  a  dark  ring,  and  the  opaque  area  (lying  outside  it)  as  a  white  ring. 
The  oval  shield  in  the  centre  also  looks  whitish,  and  in  its  axis  we  see  the  dark 
medullary  groove.     (From  Bisclmff.) 

Fig.  121.— Pear-shaped  germinal  shield  of  the  hare  (eight  days  old), 

magnified  twenty  times,     rf  medullary  groove,  fr  primitive  groove  (primitive 

mouth).      (From  KliUikcr.) 

or  "  the  first  trace  of  the  embryo."  The  primary  germinal 
layers  are  really  the  first  rudiment  of  the  embryo.  Neverthe- 
less, the  older  names  have  been  retained  in  great  measure  to 
our  own  time,  thanks  to  the  authority  of  Baer  and  Bischoff. 
Thus,  Kolliker,  for  instance,  one  of  the  most  distinguished 
and  influential  embryologists,  says,  even  in  the  latest  edition 
of  his  Human  Embryology  (1884)  :  "  In  the  middle  of  the 
pellucid  area  (of  the  chick)  we  get  later  on  the  first  traces  of 


EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA  289 

the  embryo";  and  in  the  blastodermic  vesicle  o(  the  hare 
■•  there  appears,  at  the  part  where  it  is  three-layered,  a  white, 
round,  opaque  spot,  the  embryonal  spot  (urea  embryonalis), 
which  is  no  other  than  the  first  outline  o(  the  embryo."  The 
misunderstanding  that  arises  from  these  and  similar  expres- 
sions has  led  to  a  number  of  serious  errors  in  explaining  the 
embryonic  structures.  In  view  of  these,  I  must  formally 
draw  up  the  following  principles  : — 

1.  The  so-called  "first  trace  of  the  embryo"  hi  the 
amniotes,  or  the  embryonic  shield  ( embiyaspis  ) ,  in  the 
centre  of  the  pellucid  area,  consists  merely  of  an  early  differen- 
tiation and  formation  of  the  middle  dorsal  parts. 

2.  Hence  the  best  name  for  it  is  "  the  dorsal  shield " 
(notaspis),  as  I  proposed  Ion y  ago. 

3.  The  i^erminative  area,  in  which  the  first  embryonal 
blood-vessels  appear  at  an  early  Stage,  is  not  opposed  as  an 
external  area  to  the  "embryo  proper,"  but  is  a  part  of  it. 

4.  In  the  same  way,  the  yelk-sac  or  the  umbilical  vesicle 
(the  "  relic  of  the  blastula  ")  is  not  a  foreign  external  appen- 
dage o(  the  embryo,  but  an  outlying  part  of  its  primitive 
gut,  an  embryonal  visceral  gland. 

5.  The  dorsal  shield  gradually  separates  from  the  i^ermina- 
tive  area  and  the  yelk-sac,  its  edges  growing  downwards  and 
folding  together  to  form  ventral  plates  (lamina  ventrales). 

(>.  The  yelk-sac  and  vessels  of  the  germinative  area,  which 
soon  spread  over  its  whole  surface,  are,  therefore,  real 
embryonal  organs,  or  temporary  parts  of  the  embryo,  and 
have  a  transitory  importance  in  connection  with  the  nutrition 
of  the  growing  later  body  ;  the  latter  may  be  called  the 
"  permanent  body  "  (menosoma)  in  contrast  to  them. 

The  relation  of  these  cenogenetic  features  of  the  amniotes 
to  the  palinijenetic  structures  of  the  older  non-amniotic 
vertebrates  may  be  expressed  in  the  following  theses:  The 
original  gastrula,  which  completely  passes  into  the  embryonic 
bodv  in  the  acrania,  cyclostoma,  and  amphibia,  is  early 
divided  into  two  parts  in  the  amniotes — the  embryonic 
shield  (embtyaspis),  which  represents  the  dorsal  outline  of 
the     permanent     body    (menosoma);     and     the     temporary 


29o  EMBRYOXIC  SHIELD  AND  GERMIXATIVE  AREA 

embryonic  organs  of  the  germinative  area  and  its  blood- 
vessels, which  soon  grow  over  the  whole  of  the  yelk-sac. 
The  differences  which  we  find  in  the  various  classes  of  the 
vertebrate  stem  in  these  important  particulars  can  only  be 
fully  understood  when  we  bear  in  mind  their  phylogenetic 
relations  on  the  one  hand,  and,  on  the  other,  the  cenogenetic 
modifications  of  structure  that  have  been  brought  about  by 
changes  in  the  rearing  of  the  young  and  the  variation  in  the 
mass  of  the  food-yelk. 

We  have  already  described  in  the  ninth  Chapter  the 
changes  which  this  polyphyletic  increase  and  decrease  of 
the  nutritive  yelk  causes  in  the  form  of  the  gastrula,  and 
especially  in  the  situation  and  shape  of  the  primitive  mouth. 
The  primitive  mouth  or  prostoma  is  originally  a  simple 
round  aperture  at  the  lower  (aboral)  pole  of  the  long  axis  ; 
its  dorsal  lip  is  above  and  ventral  lip  below.  In  the  holo- 
blastic  amphioxus  this  primitive  mouth  is  a  little  eccentric, 
or  shifted  to  the  dorsal  side  (Fig.  41).  The  aperture  increases 
with  the  growth  of  the  food-yelk  in  the  cyclostoma  and 
ganoids  ;  in  the  sturgeon  it  lies  almost  on  the  equator  of  the 
round  ovum,  the  ventral  lip  (a)  in  front  and  the  dorsal  lip  (b) 
behind  (Fig.  122  b).  In  the  wide-mouthed,  circular  discoid 
gastrula  of  the  selachii  or  primitive  fishes,  which  spreads 
quite  flat  on  the  large  food-yelk,  the  anterior  semi-circle  of 
the  border  of  the  disk  is  the  ventral,  and  the  posterior  semi- 
circle the  dorsal  lip  (Fig.  122  A).  The  amphiblastic  amphibia 
are  directly  connected  with  their  earlier  fish-ancestors,  the 
dipneusts  and  ganoids,  and  further  the  oldest  selachii 
(cestracion) ;  they  have  retained  their  total  unequal  segmenta- 
tion, and  their  small  primitive  mouth  (Fig.  122,  C,  ab)  is 
blocked  up  by  the  yelk-stopper,  lies  at  the  limit  of  the 
dorsal  and  ventral  surface  of  the  embryo  (at  the  aboral  pole 
of  its  equatorial  axis),  and  there  again  has  an  upper  dorsal 
and  a  lower  ventral  lip  (a,  b).  The  formation  of  a  large 
food-yelk  followed  again  in  the  stem-forms  of  the  amniotes, 
the  protaminotes  or  proreptilia,  descended  from  the  amphibia 
(Fig.  122  D).  But  here  the  accumulation  of  the  food-yelk  took 
place  only  in  the  ventral  wall  of  the  primitive-gut,   so  that 


EMI! R  YONIC  SHIELD  .  I  X/>  GBRMIN.  I II I  rE  .  I  RE.  1  291 

the  narrow  primitive  mouth  Lying  behind  was  forced  upwards, 
and  came  to  lie  on  the  back  of  the  discoid  " epigastrula "  in 
the  shape  of  the  "primitive  groove";  thus  (in  contrast  to  the 
case  of  the  selachii,  Fig.  122  J)  the  dorsal  lip  (b)  had  to  be 
in  front,  and  the  ventral  \\p(i/J  behind  (Fig.  122  D).  This 
feature  was  transmitted  to  all  the  amniotes,  whether  they 
retained  the  large  food-yelk  (reptiles,  birds,  and  monotremes), 
or  lost  it  by  atrophy  (the  viviparous  mammals). 


- 


Fig.  122.  -Median  longitudinal  section  of  the  gastrula  of  four 
vertebrates.  (From  RabT.)  .1  discogastrula  of  a  shark  (pristiurus).  11 
amphigastrula  of  a  sturgeon  ( accipenser ).  C amphigastrula  of  an  ampbibium 
(tritniij.  D  epigastrula  of  an  amniote  (diagram),  a  ventral,  b  dorsal  lip  o\ 
the  primitive  mouth. 

This  phvlogenetic  explanation  oi  gastrulation  and  ccelo- 
mation  and  the  comparative  Study  oi  them  in  the  various 
vertebrates  throw  a  clear  and  full  light  on  many  ontogenetic 
phenomena,  as  to  which  the  most  obscure  and  confused 
opinions  were  prevalent  thirty  years  ago.  In  this  we  see 
especially  the  high  scientific  value  o<i  the   biogenetic   law  and 


292  EMBRYONIC  SHIELD  AND  GERMINATIVE  AREA 

the  careful  separation  of  palingenetic  from  cenogenetic  pro- 
cesses. To  the  opponents  of  this  law  the  real  explanation  of 
these  remarkable  phenomena  is  impossible.  We  have  curious 
instances  of  this  lack  of  a  thorough  grasp  of  the  subject  in 
Wilhelm  His  (of  Leipzig)  and  Victor  Hensen  (of  Kiel). 
Although  these  industrious  observers  have  been  devoted  to 
the  accurate  description  of  ontogenetic  facts  for  more  than 
thirty  years,  they  have  completely  failed  to  detect  their  phylo- 
genetic  causes.  The  same  may  be  said  of  many  new  workers 
in  the  field  of  mechanical  and  experimental  embryology.  Of 
these  Hans  Driesch  particularly  deserves  notice  for  the 
obscurity  of  his  ideas  and  lack  of  a  real  grip  of  the  biogenetic 
processes.  In  his  violent  antagonism  to  the  theory  of  descent 
he  goes  as  far  as  to  say  that  all  Darwinists  have  softening 
of  the  brain,  and  that  Darwinism  is  only  the  illusion  of  a 
generation.  Driesch  has  lately  won  a  certain  regard  in 
uneducated  circles  by  foolish  expressions  of  this  kind,  and 
by  his  metaphysical  speculations  on  neo-vitalism.  This, 
however,  is  chiefly  grounded  on  the  fact  that  no  one  can  find 
any  rational  meaning  in  his  extraordinary  theories.  Both 
these  vitalistic  vagaries  and  the  supposed  simple  mechanical 
explanations  that  "  mechanical  evolutionists  "  give  of  historical 
processes  are  totally  unsatisfactory  (see  p.  46).  Here,  and  in 
every  other  part  of  embryology,  the  true  key  to  the  solution 
lies  in  phylogeny. 


TWELFTH  TABLE 

SYNOPSIS  OF  THE  COMPOSITION"  OF  THE 

AMNIOTE-EMBRYO  FROM  THE  PERMANENT 

BODY  (MENOSOMA)  AND  TEMPORARY 

EMBRYONIC  ORGANS. 


Primary  Constituents  of  the 

Secondary               Tertiary 

Amniote-embryo. 

Constituents.         Constituents. 

I.  A. 

a.  Cerebral  vesicle 

Embryonic  shield 

Dorsal  body 

and  head-plates. 

I. 

Embryaspis 

(  =  provertebrae- 

b.  Spinal    marrow 

Permanent  body. 

embryonic 

plates). 

and       proverte- 

Menosoma. 

-.pot 

Episoma 

1     brae-plates. 



(  a  rcn  embryon- 

stem  zone 

e.  Chorda       (axial 

Tin-  (small)  part 

n/is  ), 

(dorsal  shield). 

\     entoderm). 

ol  the  amniote- 

ov  "  embryonic 
rudiment,"  or 

embryo  (central  • 

part  ol'  tin' 

'•  first  trace  of 

fa..  Ventral     plates 

discogastrula) 

the  embryo." 

I.    13. 

(parietal    lateral 

that  developes 



\  entral  bodv 

plates,    somato- 

into the 

i       Remak's 

1        lateral  plates). 

pleural. 

permanent  bodv. 

'double  shield  " 

Hyposoma 

b.  Visceral    plates 

and  Baer  s 

parietal  zone 

(visceral  lateral 

shield.) 

,    (ventral  plates). 

plates,    splaneh 
*■     nopleura). 

Embryonic 
organs. 

The  (large)  part 
of  the  amniote- 
embryo  thai 

takes  no  part   in 

the  composition 
of  the  permanent 

body,  hut  tonus 
the  temporary 

••  extra-embry- 
onic    organs  of 
the  embryo. 


II.   A. 
yelk-sac. 
Leeithoma 
(saccus  vitel- 
line). 


II.    At. 

Germinative 
area, 

or  vascular  area. 
II.    A  j. 

Umbilical 
vesicle. 


II.   B. 
Primitive  urinary 

sae. 

Allantois 

(       urinary  vesicle 
ol'  the  amphibia). 


II.   B  i. 
Intrafcetal 

allantois. 
II.     B.    2. 

Extrafoetal 

allantois. 


a.  Light  area 

(  ii  rin  pellucida). 

b.  Dark  area 
(area  opaca). 

c.  Yelk-area 
(area  vitellina). 


a.  Urinary  bladder 

(  vesica  it  rin - 
a  rin  ). 

b.  Urinary  duct 
f  urachus). 

e.  Placenta. 


II.  C, 
Embryonic  mem- 
branes. 

Embryolemma. 


II.  C  i.  Amnion. 

Water  membrane 
(fbetal  sae  |. 

II.     C    2. 

Serolemma. 
Serous  membrane 
converted  into  the 
chorion  by  forma- 
tion of  villi. 
Chorion. 


rC  i.  Amniotic 
cavity 

'.      ( tunnioca'loni  ). 

C  j.  Serous  cavity 

(seroccelom). 
(      Excceloma    or 

i  nt  era  in  niotic 

cavity,  or  extra- 
foetal eeelom). 


CHAPTER   XIII. 

DORSAL  BODY  AND  VENTRAL  BODY 

Development  of  the  dorsal  shield  ( nofaspis).  Primitive  groove  (primitive 
mouth)  in  the  hind  half  and  medullary  groove  in  the  fore-half  of  the  dorsal 
shield.  Connection  of  the  two  median  grooves  by  the  medullary  visceral 
duet  or  neurenteric  canal.  Neuroporus.  The  oval  form  of  the  embryonic 
disk  changes  into  a  sandal-shape.  Differentiation  of  dorsal  body  (episoma  or 
stem-zone)and  ventral  body  (hyposoma  or  parietal  zone).  Separation  of  the 
two  by  the  lateral  furrow.  Differentiation  of  prevertebral  plates  and  lateral 
plates.  Transverse  studies  of  the  sole-shaped  amniote  embryo.  Separation 
of  the  medullary  tube  from  the  horn-plate.  Origin  of  the  closed  gastric  tube 
from  the  flat  gut-layer  of  the  embryonic  shield.  Formation  of  the  navel. 
Separation  of  the  mammal  embryonic  shield  from  the  embryonic  vesicle. 
Cutaneous  navel  and  intestinal  navel.  Formation  of  the  amnion,  the 
allantois,  and  the  umbilical  vesicle.  Similar  construction  of  dorsal  wall  and 
ventral  wall.  Fore  gut-cavity  and  pelvic-cavity.  Mouth-pit  and  anus-pit. 
Pro-renal  ducts.      First  blood-vessels. 

The  earliest  stages  of  the  human  embryo  are,  for  the  reasons 
already  given,  either  quite  unknown  or  only  imperfectly 
known  to  us.  But  as  the  subsequent  embryonic  forms  in 
man  behave  and  develop  just  as  they  do  in  all  the  other 
mammals,  there  cannot  be  the  slightest  doubt  that  the 
preceding  stages  also  are  similar.  We  have  been  able  to 
see  in  the  ccelomula  of  the  human  embryo  (Fig.  ioo),  by 
transverse  sections  through  its  primitive  mouth,  that  its  two 
ccelom-pouches  are  developed  in  just  the  same  way  as  in  the 
hare  (Fig.  99);  moreover,  the  peculiar  course  of  the  gastrula- 
tion  is  just  the  same. 

The  germinative  area  forms  in  the  human  embryo  in  the 
same  way  as  in  the  other  mammals,  and  in  the  axial  middle 
part  of  this  we  have  the  embryonic^shield  (embryaspisj,  the 
purport  of  which  we  considered  in  the  preceding  chapter. 
The  next  changes  of  the  embryonic  disk,  or  the  "embryonic 
spot"  (area  embrvonalis ),  take  place  in  corresponding 
fashion.  These  are  the  changes  we  are  now  going  to 
consider  more  closely. 

The  chief  part  of  the  oval  embryonic  shield  is  at  first  the 
^94 


nojfs.i/.  zio/ty  .ix/>  ventral  nonv 


narrow  hinder  end  ;  it  is  in  the  median  line  o\  this  that  the 
primitive  streak  appears  (Fig.  124  ps).  The  narrow  longi- 
tudinal groove  or  meridian  furrow  in  it — the  so-called 
••  primitive  groove" — is,  as  we  have  seen,  t he  primitive  mouth 


Fig.  1  23.  -Embryonic  vesicle  of  a  seven-days' old  hare  with  oval 
embryonic  shield  (ag)-  -<  seen  from  above,  B  from  the  side.  (From 
Kolliker.)  ay  dorsal  shield  (notaspis)or  embryonic  spot  (  un-u  embryonalis). 
In  li  the  upper  half  of  the  vesicle  is  made  up  of  the  two  primary  germinal 
layers,  the  lower  (up  to  e»)  only  from  the  outer  layer. 


Fig.  124.  Oval  embryonic  shield 
of  the  hare  (Fig.  124  •'  i>|'  six  days 
eighteen  hour-,  il  of  eight  days). 
(From  KSUiker.)  ps  primitive  streak, 
/>/•  primitive  groove,  arg  area  germi- 
nalis,  sw  sickle-shaped  terminal  growth. 


o(  the  gastrula.  In  the  gastrula-embryos  of  the  mammals, 
which  are  much  modified  cenogenetically,  this  cleft-shaped 
prostoma  is  lengthened  so  much  that  it  soon  traverses  the 
whole  o\  the  hinder  hall"  of  the  dorsal  shield  ;  as  we  find   in   a 


DORSAL  BODY  AND   VENTRAL  BODY 


hare-embryo  of  six  to  eight  days  (Fig.  125  pr).  The  two 
swollen  parallel  borders  that  limit  this  median  furrow  are  the 
lateral  lips  of  the  primitive  mouth,  right  and  left.  In  this 
way  the  bilateral,  dipleurous,  or  bilateral-symmetrical  type  of 
the  vertebrate  becomes  pronounced.  The  subsequent  head 
of  the  amniote  is  developed  from  the  broader  and  rounder 
fore-half  of  the  dorsal  shield. 

In  this  fore-half  of  the  dorsal  shield  a  median  furrow 
quickly    makes   its   appearance  (Fig.    125  rf).     This  is   the 

broader  dorsal 
furrow  or  medul- 
lary groove,  the 
first  structure  of 
the  central  nervous 
system.  The  two 
parallel  dorsal  or 
medullary  swel- 
lings that  enclose 
it  grow  together 
over  it  afterwards, 
and  form  the  me- 
dullarv  tube.  As 
is  seen  in  trans- 
verse sections,  it 
is  formed  only  of 
the  outer  germinal 
layer  (Figs.  139, 
140).  The  lips  of 
the  primitive 
mouth,  however,  lie,  as  we  know,  at  the  important  point 
where  the  outer  layer  bends  over  the  inner,  and  from  which 
the  two  coelom  pouches  grow  between  the  primary  germinal 
layers. 

Thus  the  median  primitive  furrow  (pr)  in  the  hind-half 
and  the  median  medullary  furrow  ( rf)  in  the  fore-half  of  the 
oval  shield  are  totally  different  structures,  although  the  latter 
seems  to  a  superficial  observer  to  be  merely  the  forward 
continuation    of    the    former.     Hence     they    were    formerly 


Fig.  125.— Dorsal  shield  (ag)  and  germinative 
area  of  a  hare-embryo  of  eight  days.    (From 

Kblliker.)    pr  primitive  groove,  //"dorsal  furrow. 


/>OA'S.l/.  BODY  AND   VENTRAL  BODY 


always  contused,  and  in  the  oldest  and  much-copied  illustra- 
tion of  the  dorsal  shield  of  the  hare  which  Bischoff  gave  in 
[842  (Fig.  120)  one  simple  longitudinal  furrow  goes  the 
whole  length  of  the  middle  line.  This  error  was  the  more 
pardonable  as  immediately  afterwards  the  two  grooves  do 
actually  connect  in  a  very  remarkable  way.  The  two  parallel 
dorsal  swellings,  which  pass  into  each  other  arch-wise  in 
front,  diverge  in  the  rear  and  embrace  the  anterior  end  of  the 
primitive  groove  (Fig.  125).  They  then  grow  together  over 
it  in  such  a  way  that  the  primitive  groove  (or  the  hindermost 
cavity  of  the  primitive  gut)  passes 
directly  into  the  closing  medul- 
lary tube.  The  point  of  transi- 
tion   is  the  remarkable   neurenteric 


Fig.  126.  Fk-  I27- 

Fig.   .-•<•.    Embryonic  shield  of  a  hare  of  eight  days.    (From    Van 

Beneden.)   ^-primitive  groove, en  canalis  neurentericus,  nk nodus  neurentertcus 

(or  •  Hensen's  knot  "),  */  head-process  (chorda). 

Fig    1 -t.    Longitudinal  section  of   the  ccelomula  of  amphioxus 

(from  the  left),     lentoderm,  d  primitive  gut,  en  medullary  duct,  n  nerve-tube, 

m  mesoderm,  s  Brst  primitive  segment,  c  ccelom-pouches.     (From  Hatschek. ) 

canal  (Fig.  127  en).  The  thickened  mass  at  the  border 
of  the  primitive  mouth,  which  surrounds  it,  is  the  neuren- 
teric knot  (or  "Hensen's  knot."  Fig.  126  nk).  The  direct 
connection  which  is  thus  established  between  the  two 
cavities  of  the  primitive  gut  and  the  medullary  tube  does  not 
last  long;  the  two  are  soon  definitely  separated  by  a  partition. 
The  enigmatic  canalis  neurentericus  is  a  very  old 
embryonic  organ,  and  of  great  phylogenetic  interest,  because 
it  arises  in  the  same  way  in  all  the  chordoma  (both  tunieates 
and  vertebrates).      In  every  case  it  touches  or  embraces  like 


DORSAL  BODY  AXD    VEXTRAL  BODY 


an  arch  the  posterior  end  of  the  chorda,  which  has  been 
developed  here  in  front  out  of  the  middle  line  of  the  primitive 
gut  (between  the  two  coelom-folds  of  the  sickle-groove) 
("head-process,"  Fig.  126  kf).  These  very  ancient  and 
strictly  hereditary  structures,  which  have  no  physiological 
significance  to-day,  deserve  (as  "rudimentary  organs  ")  our 
closest  attention.  The  tenacity  with  which  the  useless 
neurenteric  canal  has  been  transmitted  down  to  man  through 
the  whole  series  of  vertebrates  is  of  equal  interest  for  the 
theory  of  descent  in  general,  and  the  phylogeny  of  the 
chordonia  in  particular. 


Fig.  12S.  Fig.  129. 

r  •  Fig.  128.—  Longitudinal  section  of  the  ehordula  of  a  frog.    (From 

Balfour. )     nc  nerve-tube,  x  canalis  neurentericus,  al  alimentary  canal,  _vk  yelk- 
cells,  m  mesoderm. 

Fig.  129.— Longitudinal  section  of  a  frog-embryo.    (From  Goette.) 

m  mouth,  /  liver,   an  anus,   ne  canalis   neurentericus,   mc  medullary  tube,  pn 
pineal  gland  (epiphysis),  eh  chorda. 

The  connection  which  the  canalis  neurentericus  (Fig. 
127  en)  establishes  between  the  dorsal  nerve-tube  (11)  and 
the  ventral  gut-tube  ( d )  is  seen  very  plainly  in  the  amphioxus 
in  a  longitudinal  section  of  the  ccelomula,  as  soon  as  the 
primitive  mouth  is  completely  closed  at  its  hinder  end.  The 
medullary  tube  has  still  at  this  stage  an  opening  at  the 
forward  end,  the  neuroporus  (Fig.  86  np).  This  opening 
also  is  afterwards  closed.  There  are  then  two  completely 
closed  canals  over  each  other — the  medullary  tube  above  and 
the  gastric  tube  below,  the  two  being  separated  by  the 
chorda.  The  same  features  as  in  the  acrania  are  exhibited  by 
the  related  tunicates,  the  ascidia  (Plate  XVIII.,  Figs.  5,  6). 


poks.i/.  hoiiv  axd  ventral  nunv 


Again,  we  find  the  neurenteric  canal  in  just  the  same 
form  and  situation  in  the  amphibia.  A  longitudinal  section 
of  a  young  tadpole  (Fig.  i  2S)  shows  how  we  may  penetrate 
from  the  still  open  primitive  mouth  (x)  either  into  the  wide 
primitive  gut-cavity  ( al )  or  the  narrow  overlying  nerve-tube. 
A  little  later,  when  the  primitive  mouth  is  closed,  the  narrow 
neurenteric  canal  (Fig.  i2(),  ne)  represents  the  arched  connec- 
tion between  the  dorsal  medullary  canal  ( ' mc )  and  the 
ventral  gastric  canal. 

In  the  amniotes  this  original  curved  form  of  the 
neurenteric  canal  cannot  be  found  at   first,  because  here  the 


J>V 


Fig.  130. 

Figs.  130  and  131.-  Dorsal  shield 
Of  the  Chick.  (From  Bal/bur.) 
The  medullary  furrow  (me),  which  is 
not  \vt  visible  in  Ki_<.  130,  encloses 
with  it>  hinder  end  the  fore  >-nil  of  the 
primitive  groove  (pr)  in  Fig.  131. 


primitive  mouth  travels  completely  over  to  the  dorsal  surface 
of  the  gastrula,  and  is  converted  into  the  longitudinal 
furrow  we  call  the  primitive  groove.  Hence  the  primitive 
groove  (I'ig.  131  pr),  examined  from  above,  appears  to  be 
the  straight  continuation  of  the  fore-lying  and  younger 
medullary  furrow  (me).  The  divergent  hind  legs  of  the 
latter  embrace  the  anterior  end  of  the  former.  Afterwards 
we  have  the  complete  closing  of  the  primitive  mouth,  the 
dorsal  swellings  joining  to  form  the  medullary  tube  and 
growing  over  the  prostoma.     The  canalis  neurentericus  then 


DORSAL  BODY  AND   VENTRAL  BODY 


leads  directly,  in  the  shape  of  a  narrow  arch-shaped  tube  (Fig. 
132  ne),  from  the  medullary  tube  (sp)  to  the  gastric  tube  (pag ). 
Directly  in  front  of  it  is  the  latter  end  of  the  chorda  fc/ij. 


Fig.  132.— Longitudinal  section  of  the  hinder  end  of  a  chick.    (From 

Balfour.)  sp  medullar}-  tube,  connected  with  the  terminal  gut  (pag)  by  the 
neurenteric  canal  ( ne ),  ch  chorda,  pr  neurenteric  (or  Hensen's)  knot,  al  allan- 
tois,  ep  ectoderm,  hy  entoderm,  so  parietal  laj-er,  sp  visceral  layer,  ««  amis- 
pit,  am  amnion. 

While  these  important  processes  are  taking  place  in  the 
axial  part  of  the  dorsal  shield,  its  external  form  also  is 
changing.     The  oval  form  (Fig.   120)  becomes  like  the  sole 

of  a  shoe  or  sandal,  lyre- 
shaped  or  finger  biscuit- 
shaped  (Fig.  133).  The 
middle  third  does  not  grow 
in  width  as  quickly  as  the 
posterior,  and  still  less 
than  the  anterior  third  ; 
thus  the  shape  of  the  per- 
manent body  becomes 
somewhat  narrow  at  the 
waist.  At  the  same  time 
the  oval  form  of  the  ger- 
minative  area  returns  to 
a  circular  shape,  and 
the  inner  pellucid  area 
separates  more  clearly 
from  the  opaque  outer 
area  (Fig  134  a).  The 
completion  of  the  circle  in  the  area  marks  the  limit  of  the 
formation  of  blood-vessels  in  the  mesoderm. 


Fig.  133.— Germinal  area  or  ger- 
minal disk  of  the  hare  with  sole- 
shaped  embryonic  shield,  magnified 

about  ten  times.  The  clear  circular  field 
(d)  is  the  opaque  area.  The  pellucid 
area  (c)  is  lyre-shaped,  like  the  em- 
bryonic shield  itself  (  b).  In  its  axis  is 
seen  the  dorsal  furrow  or  medullary 
furrow  (a).     ( From  Bisrhoff. ) 


/XWS.I/.  /:<)/>)■  AND   VENTRAL  HODY 


fir- 


A- 


Fig.  134. 


.Medullary         ~A 

groove 


Neurenteric     ™ 


Fig.  1  j& 


Fig.  134.  —Embryo 
of  the  opossum,  sixty 
hours  old,  lour  mm.  in 
diameter.  I  From  Selenia. ) 
^'  the  globular  embryonic 
vesicle,  u  the  round  ger- 
minative  area,  h  limit  of 
the  ventral  plates,  r  dorsal 
shield,  o  its  fore  part,  u 
the  first  primitive  seg- 
ment, c/;  chorda,  chr  its 
fore-end,  />>-  primitive 
groove  (or  mouth). 

Fig.  135.  Sandal- 
shaped  embryonic 
shield  of  a  hare  of 
eight  days,  with  the 
fore  part  of  the  germina- 
tive  area  (<"'  opaque,  ap 
pellucid  area).  (From 
Kiilliker. )  rf  dorsal  fur- 
row, in  the  middle  of  the 
medullary  plate,  /;,  />r 
primitive  groove  I  mouth  1, 

Sta  dorsal  (Mem)  zone, 
pa  ventral  (parietal  I  /one. 
In  the  narrow  middle 
part  the  first  three  primi- 
tive    segments    may    be 


Fig.  136.    Human  embryo  at  the  sandal-stage,  two  mm.  long, 
d  of  the  second  week,  magnified  twenty-five  times.     (From  Count  Spec 


in.   long,  from  ih» 

1 


DORSAL  BODY  AXD    VEXTRAL  BODY 


The  characteristic  sandal-shape  of  the  dorsal  shield,  which 
is  determined  by  the  narrowness  of  the  middle  part,  and 
which  is  compared  to  a  violin,  lyre,  or  shoe  sole,  persists  for 
a  long  time  in  all  the  amniotes.  All  mammals,  birds, 
and  reptiles  have  substantially  the  same  construction  at  this 

stage,  and  even  for  a  longer 
or  shorter  period  after  the 
division  of  the  primitive  seg- 
ments into  the  ccelom-folds 
has  begun  (Fig.  1 35).  The 
human  embryonic  shield 
assumes  the  sandal-form  in 
the  second  week  of  develop- 
ment ;  towards  the  end  of 
the  week  our  sole-embryo 
has  a  length  of  about  one 
line  or  two  millimetres 
(Fig.  136).  (Cf.  Plates  IV. 
and  V.) 

The  complete  bilateral 
svmmetrv  of  the  vertebrate 
bodv  is  very  early  indicated 
in  the  oval  form  of  the  em- 
bryonic shield  (Fig.  120)  by 
the  median   primitive  streak  ; 

in   the  sandal-form   it  is  even 
Fig.    137.  —  Sandal-shaped   em- 
bryonie  shield  of  a  hare  of  nine    more  pronounced  (Figs.  134- 

days.      (From    Kiillikcr.)      (Back    view  „,         T1                -    .                              r 

from  above.  I    sts  stem-zone  or  dorsal  I3b>-       -1  lle    axial     Organs    Ot 

shield  (with  eight  pairs  of  primitive  seg-  the   mjdclle   plane  (the  primi- 

merits),  ps  parietal  or  ventral  zone,  tip  l               \           r 

pellucid  area,  af  amnion-fold,  h  heart,    tive   streak   behind,   the   me- 

p/i    pericardial     cavity,     t«     omphalo-  . 

mesenteric  vein,  n/,  eye-vesicles,  ;•/;  tore  dtlllary  tube  in  front,  and  the 

brain,  ink   middle  brain,  ////  hind  brain,         ,  ,  j„       „„,.u\ ^<-.'ll 

uw  primitive  segments  (or  vertebrae).  chorda    underneath)    are    Still 

more  clearly  differentiated  in 
the  sole-shaped  embryonic  shield,  and  so  are  the  lateral 
organs  that  develop  symmetrically  to  the  right  and  left  of 
them.  In  these  lateral  organs  of  the  embryonic  shield  a 
darker  central  and  a  lighter  peripheral  zone  become  more 
obvious  ;    the  former  is  called  the  stem-zone  (Fig.  137  stz), 


SANDAL-EMBRYOS  OF  SAUROPSIDA 


The  Evolution  of  Man.   1'.  Ed. 


PL  IV 


SANDAL-EMBRYOS  OF  MAMMALS 


The  Evolution  of  Man.   V.Ed. 


PL  V. 


U   Man 

(homo) 


DORSAL  BODY  AND   VENTRAL  BODY 


and  the  latter  the  parietal  zonefps);  from  the  first  we  gel 
the  dorsal  and  from  the  second  the  ventral  half  of  the 
body-wall. 

The  stem-zone  o(  the  amniote  embryo  would  be  called 
more  appropriately  the  dorsal  zone  or  dorsal  shield  ;  from  it 
developes  the  whole  of  the  dorsal  half  o(  the  later  body  (or 
permanent  body) — that  is  to  say,  the  dorsal  body  (episoma). 
Again,  it  would  be  better  to  call  the  "parietal  zone"  the 
ventral  zone  or  ventral  shield  ;  from  it  develop  the  ventral 
"lateral  plates,"  which  afterwards  separate  from  the  embryonic 
vesicle  and  form  the  ventral  body  (hypotonia ) — that  is  to  say, 
the  ventral  half  of  the  permanent  body,  together  with  the 
body-cavity  and  the  gastric  canal  that  it  encloses. 

The  sole-shaped  germinal  shields  of  all  the  amniotes  are 
still,  at  the  Stage  of  construction  which  Fig.  [37  illustrates  in 
the  hare  and  Fig.  i,yS  in  the  opossum,  so  like  each  other  that 
we  can  either  not  distinguish  them  at  all  or  only  by  means  of 
quite  subordinate  peculiarities  in  the  size  of  the  various  parts. 
Moreover,  the  human  sandal-shaped  embryo  cannot  at  this 
stage  be  distinguished  from  those  of  other  mammals,  and  it 
particularly  resembles  that  of  the  hare.  I  have  given  on 
Plates  IV.  and  V.  the  sandal-shaped  embryos  of  six  different 
amniotes  for  the  purpose  of  comparison,  and  have  reduced 
them  to  the  same  size  ;  all  of  them  are  highly  magnified. 
Plate  IV.  shows  the  sandal-shaped  embryonic  shield  (at  three 
stages  of  development)  of  three  of  the  sauropsids  :  E  lizard 
(lacerta),  C  tortoise  (chelonia),  II  hen  (gallus).  Plate  V. 
gives  the  embryos  of  three  mammals:  5"  pig  (sits).  A'  hare 
(lepus),  M  man  (homo). 

On  the  other  hand,  the  outer  form  of  these  Hat  sandal-shaped 
embryos  is  very  different  from  the  corresponding  form  oi  the 
holoblastic  lower  animals,  especially  the  acrania  (amphioxus). 
Nevertheless,  the  body  is  just  the  same  in  the  essential 
features  of  its  structure  as  that  we  find  in  the  chordula  ol'  the 
latter  (Figs.  NO -89),  and  in  the  segmented  embryonic  forms 
which  immediately  develop  from  it.  The  striking  external 
difference  is  here  again  due  to  the  fact  that  in  the  palingenetic 
embryos   o(  the   amphioxus  (Figs!  80,  87)  and    the   amphibia 


DORSAL  BODY  AXD   VEXTRAL  BODY 


(Figs.  88.  89)  the  gut-wall  and  body-wall  form  closed  tubes 
from  the  first,  whereas  in  the  cenogenetic  embryos  of  the 
amniotes  they  are  forced  to  expand  leaf-wise  on  the  surface 
owing:  to  the  great  extension  of  the  food-velk. 


Fig.  138.—  Sandal-shaped  embryonic  shield  of  an  opossum  (dideU 

phys),  throe  days  old.  (From  Selenia.)  (Back  view  from  above.)  s/c  stem- 
zone  or  dorsal  shield  (with  eight  pairs  of  primitive  segments),  ps  parietal  or 
ventral  zone,  ap  pellucid  area,  no  opaque  area,  /;/;  halves  of  the  heart,  v  lore- 
end,  //  hind-end.  In  the  median  line  we  see  the  chorda  (ch)  through  the 
transparent  medullary  tube  (m).  11  primitive  segment,  pr  primitive  streak 
(or  primitive  mouth). 

It  is  all  the  more  notable  that  the  early  separation  of 
dorsal  and  ventral  halves  takes  place  in  the  same  rigidly 
hereditary  fashion  in  all  the  vertebrates.  In  both  the  acrania 
and  the  craniota  the  dorsal  body  is  about  this  period  separated 


nOA'S.l/.  /,'(>/> V  .l.\/>   VENTRAL  BODY 


from  the  ventral  body.     In  the  middle  part  o(  the  body  this 

division  has  already  taken  place  by  the  construction  of  the 
axial  chorda  between  the  dorsal  nerve-tube  and  the  ventral 
canal.  But  in  the  outer  or  lateral  part  of  the  body  it  is  only 
brought  about  by  the  division  o(  the  coelom-pouches  into  two 
sections  by  a  frontal  constriction — a  dorsal  episomite  (dorsal 
segment  or  provertebra)  and  a  ventral  hyposomite  (or  ventral 
segment).  In  the  amphioxus  each  of  the  former  makes  a 
muscular  pouch,  and  each  of  the  latter  a  sex-pouch  or 
gonad.  (Cf.  the  transverse  section  of  the  vertebrate. 
Figs.  104,  105,  and  Figs.  3-7  on  Plate  VI.) 

These  important  processes  of  differentiation  in  the 
mesoderm,  which  we  will  consider  more  closely  in  the 
next  Chapter,  proceed  step  by  step  with  interesting  changes 
in  the  ectoderm,  while  the  entoderm  changes  little  at 
first.  We  can  study  these  processes  best  in  transverse 
sections,  made  vertically  to  the  surface  through  the  sole- 
shaped  embryonic  shield.  Such  a  transverse  section  of  a 
chick-embryo,  at  the  end  of  the  first  day  of  incubation,  shows 
the  gut-gland  layer  as  a  very  simple  epithelium,  which  is 
spread  like  a  leaf  over  the  outer  surface  of  the  food-yelk 
(Fig.  139  tltl).  The  chorda  (ch)  has  separated  from  the 
dorsal  middle  line  of  the  entoderm  ;  to  the  right  and  left  of  it 
are  the  two  halves  of  the  mesoderm,  or  the  two  ccelom-folds. 
A  narrow  cleft  in  the  latter  indicates  the  body-cavity  ( inch ) ; 
this  separates  the  two  plates  of  the  ccelom-pouches,  the  lower 
(visceral)  and  upper  (parietal).  The  broad  dorsal  furrow 
(Rf)  formed  by  the  medullary  plate  (in)  is  still  wide  open, 
but  is  divided  from  the  lateral  horn-plate  ( It )  by  the  parallel 
medul la r v  swell i n gs. 

As  the  medullary  swellings  rise  and  bend  towards  each 
other  (Fig.  140  in),  one  of  these  parallel  longitudinal  furrows, 
the  lateral  furrow  (sulcus  lateralis),  is  formed  in  the  mesoderm 
on  each  side.  In  this  lateral  furrow  we  find  at  first  the  pro- 
renal  duct  (Fig.  141  ung).  As  the  lateral  furrow  cuts  com- 
pleted through  the  middle  layer,  this  falls  into  two  sections  : 
the  inner  or  middle  part  ( 11 )  is  the  primitive  segment  piece, 
which  forms  the  greater  part  of  the  Stem-zone,  and  afterwards 


DORSAL  BODY  AND    VENTRAL  BODY 


divides  by  articulation  into  the  chain  of  somites  (in  Figs. 
137  and  1 38  with  eight  pairs  of  somites  already).  The  outer 
or  lateral  section  is  the  lateral  plate  (Fig.  140  sp)  ;  when  we 
look  at  it  from  above  it  appears  as  the  parietal  zone,  and 
afterwardsjdivides  into  the  two  fibrous  layers.  In  the  fore 
half  of  the  embryonic  shield,  which  corresponds  to  the  later 


Fig.  139.— Transverse  section  of  the  embryonic  shield  of  a  chiek, 

at  the  end  of  the  first  (.lay  of  incubation).  (From  Kolliier.)  h  horn-plate, 
m  medullary  plate,  forming  the  dorsal  furrow  (Rf),  ch  chorda,  uwh  ccelom-cleft, 

mvp  dorsal  part  of  the  mesoderm,  sp  ventral  part  (lateral  plates),  dd  gut-gland 
layer. 

head,  there  is  no  separation  between  the  inner  provertebral 
mass  and  the  outer  lateral  plates.  The  median  innermost 
part  of  the  lateral  plates,  which  touches  the  primitive  segment 
piece  or  provertebral  plate,  is  called  the  middle  plate  (Fig.  141, 
nip).  Underneath  it  we  find  the  first  two  blood-vessels,  the 
primitive  aortas  ( ' ao ). 

During  these  processes  important  changes  are  taking 
place  in  the  outer  germinal  layer  (the  "skin-sense  layer"). 
The  continued  rise  and  growth  of  the  dorsal  swellings  causes 


Fig.  140.— Transverse  section  of  the  embryonic  disk  of  a  ehiek  at 

tlie  end  of  the  first  day  of  incubation,  a  little  more  advanced  than  Fig.  139, 
magnified  about  twenty  times.  The  edges  of  the  medullary  plate  (m).  the 
medullary  swellings  (w),  which  separate  the  medullary  from  the  horn-plate 
( h ).  are  bending  towards  each  other.  At  each  side  of  the  chorda  (ch)  the 
primitive  segment  plates  (u)  have  separated  from  the  lateral  plates  (sp).  A 
gut-gland  layer.      (From  Remak.) 

their  higher  parts  to  bend  together  at  their  free  borders, 
approach  nearer  and  nearer  (Fig.  140  w),  and  finally  unite. 
Thus  in  the  end  we  get  from  the  open  dorsal  furrow,  the 
upper  cleft  of  which  becomes  narrower  and  narrower,  a  closed 
cylindrical  tube  (Fig.  141  nir).  This  tube  is  of  the  utmost 
importance  ;    it   is  the  first  rudiment  of  the  central   nervous 


DOh'SM.  /;<)/>)■  AND  VENTRAL  BODY 


system,  the  brain  and  spinal  marrow,  the  medullary  tube 
(tubus  mednllaris ).  This  ontogenetic  fact  was  formerly 
looked  upon  as  very  mysterious.  We  shall  sec  presently 
thai  in  the  light  o(  the  theory  o\  descent  it  is  a  thoroughly 
natural  process.  The  phylogenetic  explanation  oi~  it  is  that 
the  central  nervous  system  is  the  organ  by  means  of  which 
all  intercourse  with  the  outer  world,  all  psychic  action  and 
sense-perception,  are  accomplished  ;  hence  it  was  bound  to 
develop  originally  from  the  outer  and  upper  surface  of  the 
body,  or  from  the  epidermis.  The  medullary  tube  afterwards 
separates  completely  from  the  outer  germinal  layer,  and  is 
surrounded  by  the  middle  parts  of  the  provertebrae  and 
forced  inwards  (Fig.  151).  The  remaining  portion  of  the 
skin-sense  layer  (Fig.  141  //)  is  now  called  the   horn-plate  or 


/.,/ 


eA  u*r        ao        *'p        J  J      iff 

Fig.  141.  -Transverse  section  of  the  embryonic  shield  (of  a  chick, 
on  the  second  day  o(  incubation),  magnified  about  one  hundred  times.  (From 
KoUiier.)  h  horn-plate,  mr  medullary  tube,  ung  prerenal  duct,  «;.•  primitive 
segments,  ///>/  skin-fibre  layer,  «//>  middle  plate,  df  gut-fibre  layer,  sp  coelom- 
folds,  no  primitive  aorta,  dd gut-gland  layer. 

horn-layer,  because  from  it  is  developed  the  whole  of  the 
outer  skin  or  epidermis,  with  all  its  horny  appendages  (nails, 
hair,  etc.).     (Cf.  Plates  VI.  and  VII.  and  the  explanation.) 

A  totally  different  organ,  the  prorenal  (primitive  kidney) 
duct  ( ung J,  is  found  to  be  developed  at  an  early  stage  from 
the  ectoderm.  This  is  originally  a  quite  simple,  tube-shaped, 
lengthy  duct,  or  straight  canal,  which  runs  from  front  to  rear 
at  each  side  of  the  provertebrae  (on  the  outer  side,  Fig.  141, 
ung).  It  originates,  it  seems,  out  o(  the  horn-plate  at  the 
side  oi  the  medullary  tube,  in  the  gap  that  we  find  between 
the  prevertebral  and  the  lateral  plates.  The  prorenal  duct  is 
visible  in  this  gap  even  at  the  time  of  the  severance  o\  the 
medullary  lube  from  the  horn-plate.  Other  observers  think 
that  the  first  trace  oi  it  does  not  come  from  the  skin-sense 
layer,  but  the  skin-fibre  layer. 


DORSAL  BODY  AA'D   VENTRAL  BODY 


The  inner  germinal  layer,  or  the  gut-fibre  layer  (Fig.  141 
dd),  remains  unchanged  during  these  processes.  A  little 
later,  however,  it  shows  a  quite  flat,  groove-like  depression 
in  the  middle  line  of  the  embryonic  shield,  directly  under  the 
chorda.  This  depression  is  called  the  gastric  groove  or 
furrow.  This  at  once  indicates  the  future  lot  of  this  germinal 
layer.  As  this  ventral  groove  gradually  deepens,  and  its 
lower  edges  bend  towards  each  other,  it  is  formed  into  a 
closed  tube,  the  alimentary  canal,  in  the  same  way  as  the 
medullary  groove  grows  into  the  medullary  tube.  The  gut- 
fibre  layer  (Fig.  142  /),  which  lies  on  the  gut-gland  layer  (d), 
naturally  follows  it  in  its  folding.  Moreover,  the  incipient 
gut-wall  consists  from  the  first  of  two  layers,  internally  the 
gut-gland  layer  and  externally  the  gut-fibre  layer. 

The  formation  of  the  alimentary  canal  resembles  that  of 
the  medullary  tube  to  this  extent — in  both  cases  a  straight 
groove  or  furrow  arises  first  of  all  in  the  middle  line  of  a  flat 
layer.  The  edges  of  this  furrow  then  bend  towards  each 
other,  and  join  to  form  a  tube  (Fig.  142).  But  the  two 
processes  are  really  very  different.  The  medullary  tube 
closes  in  its  whole  length,  and  forms  a  cylindrical  tube, 
whereas  the  alimentary  canal  remains  open  in  the  middle, 
and  its  cavity  continues  for  a  long  time  in  connection  with 
the  cavity  of  the  embryonic  vesicle.  The  open  connection 
between  the  two  cavities  is  only  closed  at  a  very  late  stage, 
the  construction  of  the  navel.  The  closing  of  the  medullary 
tube  is  effected  from  both  sides,  the  edges  of  the  groove 
joining  together  from  right  and  left.  But  the  closing  of  the 
alimentary  canal  is  not  only  effected  from  right  and  left,  but 
also  from  front  and  rear,  the  edges  of  the  ventral  groove 
growing  together  from  every  side  towards  the  navel. 
Throughout  the  three  higher  classes  of  vertebrates  the  whole 
of  this  process  of  the  secondary  construction  of  the  gut  is 
closely  connected  with  the  formation  of  the  navel,  or  with  the 
separation  of  the  embryo  from  the  yelk-sac  or  umbilical 
vesicle.     (Cf.  Fig.  108,  and  Plate  VII.,  Figs.  14,  15.) 

In  order  to  get  a  clear  idea  of  this,  we  must  understand 
carefully  the  relation  of  the  embryonic  shield  to  the  germinative 


DORSAL  BODY  AND  VENTRAL  HOI'Y 


area  and  the  embryonic  vesicle.  This  is  done  best  by  a 
comparison  of  the  five  stages  which  are  shown  in  longitudinal 

section  in  Pigs.  143-147.  The  embryonic  shield  fc),  which 
at  first  projects  very  slightly  over  the  surface  o(  the  germina- 
tive  area,  soon  begins  to  rise  higher  above  it,  and  to  separate 
from  the  embryonic  vesicle.     At   this  point   the   embryonic 

shield,  looked  at  from  the  dorsal  surface,  shows  still  the 
original  simple  sandal-shape  (Figs.  135-138).  We  do  not  yet 
see  any  trace  oi  articulation  into  head,  neck,  trunk,  etc.,  or 
limbs.      But   the   embryonic   shield   has   increased   greatly   in 


Fig.  14-—  Three  diagrammatic  transverse  sections  of  the  em- 
bryonic disk  of  the  higher  vertebrate,  to  show  the  origin  of  the  tubular 
organs  from  the  bending  germinal  layers.  In  Fig.  A  the  medullary  tube  (n J 
and  the  alimentary  canal  ( a  J  are  still  open  grooves.  In  Fig.  B  the  medullary 
tube  (n)  and  the  dorsal  wall  are  closed,  but  the  alimentary  canal  (a)  and  the 

ventral  wall  are  closed  ;  the  prerenal  duets  (  it )  mc  cut  oil'  from  the  horn-plate 
(  h  )  and  internally  connected  with  segmental  prerenal  canals.  Iii  Fig.  C  both 
the  medullary  tube  and  the  dorsal  wall  above  and  the  alimentary  canal  and 
ventral  wall  below  are  elosed.  All  the  open  grooves  have  become  closed 
tubes  ;  the  primitive  kidneys  are  directed  inwards.  The  figures  have  the  same 
meaning  in  all  three  figures:  /;  skin-sense  layer,  n  medullary  tube,  n  prerenal 
duets,  .r  axial  rod,  i  primitive-vertebra,  r  dorsal  wall,  b  ventral  wall,  r  body- 
cavity  or  coeloma,  /'  gut-fibre  layer,  /  primitive  artery  (aortal.  -.<  primitive  vein 
(subin  testinal  vein),  d gut-fibre  layer,  a  alimentary  canal.  (Cf.  Plates  VI.  and 
VII.) 

thickness,  especially  in  the  anterior  part.  It  now  has  the 
appearance  of  a  thick,  oval  swelling,  strongly  curved  over  the 
surface  of  the  germinative  area.  It  begins  to  sever  com- 
pletely from  the  embryonic  vesicle,  with  which  it  is  connected 
at  the  ventral  surface.  As  this  severance  proceeds,  the  back 
bends  more  and  more;  in  proportion  as  the  embryo  grows 
the  embryonic  vesicle  decreases,  and  at  last  it  merely  hangs 
as  a  small  vesicle  from  the  belly  of  the  embryo  (Fig.  147  <A  I. 
In  consequence  of  the  growth-movements  which  cause  this 


DORSAL  BODY  AX D    VEXTRAL  BODY 


Fig. 

'43- 


Fig. 
"45- 


Fig. 
144. 


Fig. 

,46. 


Figs.    143-147.— Five  diagrammatic    longitudinal    sections  of  the 
maturing  mammal  embryo  and  its  envelopes.    In  Figs.  143-146  the 

longitudinal  section  passes  through  the  sagittal  or  middle  plane  of  the  body, 
dividing  the  right  and  left  halves;  in  Fig.  147  the  embryo  is  seen  from  the  left 
side.  In  Fig.  143  the  tufted  prochorion  ( 1I1/'  )  encloses  the  germinal  vesicle, 
the  wall  of  which  consists  of  the  two  primary  layers.  Between  the  outer  (a) 
and  inner  (i)  layer  the  middle  layer  (tn)  has  been  developed  in  the  region  of 
the  gorminative  area.  In  Fig.  144  the  embryo  (e)  begins  to  separate  from 
the  embryonic  vesicle  ( 'ds ),  while  the  wall  of  the  amnion-fold  rises  about  it  (in 
front  as  head-sheath,  is,  behind  as  tail-sheath,  ss).  In  Fig-,  145  the  edges  of 
the  amniotic  fold  (am)  rise  together  over  the  back  of  the  embryo,  and  form 
the  amniotic  cavity  ( ah)  ;  as  the  embryo  separates  more  completely  from  the 
embryonic  vesicle  (ds)  the  alimentary  canal  (dd)  is  formed,  from  the  hinder 
end  of  which  the  allantois  grows  (al).      In  Fig.   146  the  allantois  is  larger  ;   the 


/XM'.s'.!/.  /;/)/>]■  .l.\/>   VENTRAL  HODY 


severance,  a  groove-shaped  depression  is  formed  at  the  surface 

of  the  vesicle,  the  limiting  furrow,  which  surrounds  the  vesicle 
in  the  shape  of  a  pit,  and  a  circular  mound  or  dam 
(Fig.  144  ks)  is  formed  at  the  outside  of  this  pit  by  the  eleva- 
tion of  the  contiguous  parts  of  the  germinal  vesicle. 

In  order  to  understand  clearly  this  important  process,  we 
may  compare  the  embryo  to  a  fortress  with  its  surrounding 
rampart  and  trench.  The  ditch  consists  of  the  outer  part  of 
the  germinative  area,  and  comes  to  an  end  at  the  point  where 
the  area  passes  into  the  vesicle.  The  important  fold  of  the 
middle  germinal  layer  that  brings  about  the  formation  of  the 
body-cavity  proceeds  peripherally  beyond  the  borders  of  the 
embryo  over  the  whole  germinative  area.  At  first  this  middle 
layer  reaches  as  far  as  the  germinative  area  ;  the  whole  of  the 
rest  of  the  embryonic  vesicle  consists  in  the  beginning  only  of 
the  two  original  limiting  layers,  the  outer  and  inner  germinal 
layers.  Hence,  as  far  as  the  germinative  area  extends  the 
germinal  layer  splits  into  the  two  plates  we  have  already 
recognised  in  it,  the  outer  skin-fibre  layer  and  the  inner  gut- 
fibre  layer.  These  two  plates  diverge  considerably,  a  clear 
fluid  gathering  between  them  (Fig.  145  am).  The  inner 
plate,  the  gut-fibre  layer,  remains  on  the  inner  layer  oi~  the 
embryonic  vesicle  (on  the  gut-gland  layer).  The  outer  plate, 
the  skin-fibre  layer,  lies  close  on  the  outer  layer  o(  the 
germinative  area,  or  the  skin-sense  layer,  and  separates 
together  with  this  from  the  embryonic  vesicle.  From  these 
two  united  outer  plates  is  formed  a  continuous  membrane. 
This  is  the  circular  mound  that  rises  higher  and  higher 
round  the  whole  embryo,  and  at  last  joins  above  it  (Figs.  144- 
147  am).     To   return    to  our   illustration   o(   the    fortress,    we 


yrelk-sac  ( ds )  smaller.  In  Fig.  1 47  1  In-  embryo  shows  the  gill-clefts  and  the 
outline  of  tlu'  two  legs;  the  chorion  has  formed  branching  villi  Hurts).  In  all 
lour  figures  *'  embryo,  "  outer  germinal  layer,  m  middle  germinal  layer, 
1  inner  germinal  layer,  am  amnion  lis  head-sheath,  ss  tail-sheath),  ah  amniotic 
cavity,  as  amniotic  sheath  of  the  umbilical  cord,  kh  embryonic  vesicle,  ds  yolk- 
sac  (umbilical  vesicle),  o^p vitelline  duct,  <//  gut-fibre  layer,  dd gut-gland  layer, 
al  allantois,  :■/  -hh  place  o(  heart,  <l  vitelline  membrane  (ovolemma  or  procho- 
riom,  </•  tufts  or  villi  <\\  same,  sh  serous  membrane  (serolemma),  s*  tufts  oi 
same,  ch  chorion,  ch*  tufts  or  villi,  si  terminal  vein,  r  pericoelom  or  serocoelom 
(tin-  space,  tilled  with  fluid,  between  the  amnion  ami  chorion).  (  From  KSUiker.  1 
(Cf.  Plate  VII.,  Figs,  14  and  15.) 


DORSAL  BODY  AND   VENTRAL  BODY 


must  imagine  the  circular  rampart  to  be  extraordinarily  high 
and  towering  far  above  the  fortress.  Its  edges  bend  over 
like  the  combs  of  an  overhanging  wall  of  rock  that  would 
enclose  the  fortress  ;  they  form  a  deep  hollow,  and  at  last 
join  together  above.  In  the  end  the  fortress  lies  entirely 
within  the  hollow  that  has  been  formed  by  the  growth  of 
the  edges  of  this  large  rampart.  (Cf.  Figs.  148-152  and 
Plate  VII.,  Fig.  14.) 

As  the  two  outer  layers  of  the  germinative  area  thus  rise 
in  a  fold  about  the  embryo,  and  join  above  it,  they  come  at 
last  to  form  a  spacious  sac-like  membrane  about  it.  This 
envelope  takes  the  name  of  the  germinative  membrane,  or 
water-membrane,  or  amnion  (Fig.  147  am).  The  embryo 
floats  in  a  watery  fluid,  which  fills  the  space  between  the 
embryo  and  the  amnion,  and  is  called  the  amniotic  fluid 
(Figs.  146,  147  ah).  We  will  deal  with  the  significance  of 
this  remarkable  formation  later  on  (Chapter  XV.).  For  the 
moment  it  does  not  interest  us,  as  it  has  no  direct  relation  to 
the  construction  of  the  body. 

Among  the  various  appendages  which  we  shall  have  to 
discuss  later  we  will  only  mention,  in  passing,  the 
allantois  and  the  yelk-sac.  The  allantois,  or  the  urinary 
sac  (Figs.  145,  146  a/),  is  a  pear-shaped  vesicle  that  grows 
from  the  hindermost  part  of  the  alimentary  canal  ;  its 
outermost  section  forms,  with  its  vessels,  the  foundation 
of  the  placenta.  In  front  of  the  allantois  the  yelk-sac  or 
umbilical  vesicle  (ds),  the  remainder  of  the  original 
embryonic  vesicle,  starts  from  the  open  belly  of  the 
embryo  (Fig.  143  kh).  In  more  advanced  embryos,  in 
which  the  gastric  wall  and  the  ventral  wall  are  nearly 
closed,  it  hangs  out  of  the  navel-opening  in  the  shape  of 
a  small  vesicle  with  a  stalk  (Figs.  146,  147,  ds).  Its  wall 
consists  of  two  layers,  the  gut-gland  layer  within  and  the 
gut-fibre  layer  without.  Hence  it  is  a  vesicular  appendage 
of  the  alimentary  canal  proper,  an  embryonic  "gastric 
gland."  The  more  the  embryo  grows,  the  smaller  becomes 
the  vitelline  (yelk)  sac  or  lecithoma.  At  first  the  embryo 
looks  like  a  small  appendage  of  the  large  embryonic  vesicle. 


DORSAL  BODY  AND   VENTRAl    BODY 


Afterwards  it  is  the  yolk-sac,  or  the  remainder  of  the 
embryonic  vesicle,  that  seems  a  small  pouch-like  appendage 
o(  the  embryo  (Fig.  147  ds).  It  ceases  to  have  any  signifi- 
cance in  the  end.  The  very  wide  opening,  through  which 
the  gastric  cavity  at  first  communicates  with  the  umbilical 
vesicle,  becomes  narrower  and  narrower,  and  at  last  disappears 
altogether.  The  nave/,  the  small  pit-like  depression  that  we 
find  in  the  developed  man  in  the  middle  of  the  abdominal 
wall,  is  the  spot  at  which  the  remainder  of  the  embryonic 
vesicle  (the  umbilical  vesicle)  originally  entered  into  the 
ventral  cavity,  and  joined  on  to  the  growing  gut.  (Cf. 
Figs.  14  and  15  on  Plate  \TI.) 

The  origin  of  the  navel  coincides  with  the  complete 
closing  o(  the  external  ventral  wall.  In  the  amniotes  the 
ventral  wall  originates  in  the  same  way  as  the  dorsal  wall. 
Both  are  formed  substantially'  from  the  skin-fibre  layer,  and 
externally  covered  with  the  horn-plate,  the  peripheral  section 
of  the  skin-sense  layer.  Both  come  into  existence  by  the 
conversion  of  the  four  flat  germinal  lavers  of  the  embryonic 
shield  into  a  double  tube  by  folding  from  opposite  directions  ; 
above,  at  the  back,  we  have  the  vertebral  canal  which 
encloses  the  medullary  tube,  and  below,  at  the  belly,  the 
wall  of  the  body-cavity  which  contains  the  alimentary-  canal 
(Fig.  .42). 

We  will  consider  the  formation  of  the  dorsal  wall  first 
and  that  of  the  ventral  wall  afterwards  (Figs.  14N-152).  In  the 
middle  of  the  dorsal  surface  of  the  embryo  there  is  originally-, 
as  we  already  know,  the  medullary  f»irj  tube  directly  under- 
neath the  horn-plate  (h)  from  the  middle  part  of  which  it  has 
been  developed.  Later,  however,  the  prevertebral  plates  ( uw ) 
grow  over  from  the  right  and  left  between  these  originally 
connected  parts  (Figs.  150,  151).  The  upper  and  inner 
edges  of  the  two  prove rtebral  plates  push  between  the  horn- 
plate  and  medullary  tube,  force  them  away  from  each  other, 
and  finally  join  between  them  in  a  seam  that  corresponds  to 
the  middle  line  of  the  back.  The  coalescence  of  these  two 
dorsal  plates  and  the  closing  in  the  middle  of  the  dorsal  wall 
take  place  in   the  same  way  as  the   medullary  tube,  which    is 


DORSAL  BODY  AND   VEXTRAL  BODY 


henceforth  enclosed  by  the  vertebral  tube.  Thus  is  formed 
the  dorsal  wall,  and  the  medullary  tube  takes  up  a  position 
inside  the  body.  In  the  same  way  the  provertebral  mass 
grows  afterwards  round  the  chorda,  and  forms  the  vertebral 
column.  Below  this  the  inner  and  outer  edge  of  the  pro- 
vertebral  plate  splits  on  each  side  into  two  horizontal  plates, 
of  which  the  upper  pushes  between  ihe  chorda  and  medullary 
tube,  and  the  lower  between  the  chorda  and  srastric  tube.     As 


Fig.  149. 
Figs.  148-151.— Transverse  sections  of  embryos  (of  chicks).    Fig.  148 

of  the  second,  Fig'.  149  of  the  third,  Fig.  150  of  the  fourth,  and  Fig.  151  of  the 
fifth  day  of  incubation.  Pig's.  14S-150  from Kolliker,  magnified  about  [00 times  ; 
Fig.  151  from  Remak,  magnified  about  twenty  times,  h  horn-plate,  mr  medul- 
lary tube,  uiig-  prerenal  duct,  un  prerenal  vesicles,  hf>  skin-fibre  layer, 
m  =  mu  —  mp  muscle-plate,  itiv  provertebral  plate  [wh  cutaneous  rudiment  of  the 
body  of  the  vertebra,  ivb  of  the  arch  of  the  vertebra,  ivq  the  rib  or  transverse 
continuation),  uwh  provertebral  cavity,  ch  axial  rod  or  chord, sh  chorda-sheath, 
bh  ventral  wall,  g  hind  and  v  fore  root  of  the  spinal  nerves,  a=af=am  amniotic 
fold,  p  body-cavity  or  cceloma,  c(/"gut-fibre  layer,  ao  primitive  aortas,  sa  secon- 
dary aorta,  vr  cardinal  veins,  d=dd  gut-gland  layer,  dr  gastric  groove.  In 
Fig.  14S  the  larger  part  of  the  right  half,  in  Fig.  149  the  larger  part  of  the  left 
half,  of  the  section  is  emitted.  Of  the  velk-sac  or  remainder  of  the  embrvonic 
vesicle  only  a  small  piece  of  the  wall  is  indicated  below.  (Cf.  the  sections  in 
Plate  VI.,  Figs.  3-S.) 


nOKSA/.  /{()/))■  .l.V/>  VENTRAL  />■()/>)' 


Fig.   rS 


the  plates  meet  from  both  sides  above  and  below  the  chorda, 
they  completely  enclose  it,  and  so  form  the  tubular,  outer 
chord-sheath,   the    skeleton-forming  sheath    from    which    the 


316  DORSAL  BODY  AXD   VEXTRAL  BODY 

vertebral  column  is  formed  {perichorda,  Fig.  142  C,  s ;  Figs. 
150  wh,  151).  (Cf.  Figs.  3-8  on  Plate  VI.  and  the  following 
Chapters.) 

We  find  below  in  the  construction  of  the  ventral  wall 
precisely  the  same  processes  as  in  the  formation  of  the  dorsal 
wall  (Fig.  142  b,  Fig.  149  hp,  Fig.  151  bh).  It  is  formed  on 
the  flat  embryonic  shield  of  the  amniotes  from  the  upper 
plates  of  the  parietal  zone,  or  the  parietal  lamella  of  the 
lateral  plates,  which  is  covered  with  the  horn-plate.  The 
right  and  left  parietal  plates  bend  downwards  towards  each 
other,  and  grow  round  the  gut  in  the  same  way  as  the  gut 
itself  closes.  The  outer  part  of  the  lateral  plates  forms  the 
ventral  wall  or  the  lower  wall  of  the  body,  the  two  lateral 
plates  bending  considerably  on  the  inner  side  of  the  amniotic 
fold,  and  growing  towards  each  other  from  right  and  left. 
While  the  alimentary  canal  is  closing,  the  body-wall 
also  closes  on  all  sides.  Hence  the  ventral  wall,  which 
embraces  the  whole  ventral  cavity  below,  consists  of  two 
parts,  two  lateral  plates  that  bend  towards  each  other. 
These  approach  each  other  all  along,  and  at  last  meet  at  the 
navel.  We  ought,  therefore,  really  to  distinguish  two 
navels,  an  inner  and  an  outer  one.  The  internal  or  intestinal 
navel  is  the  definitive  point  of  the  closing  of  the  alimentary 
wall,  wrhich  puts  an  end  to  the  open  communication  between 
the  ventral  cavity  and  the  cavity  of  the  yelk-sac  (Fig.  108). 
The  external  or  cutaneous  navel  is  the  definitive  point  of  the 
closing  of  the  ventral  wall ;  this  is  visible  in  the  developed 
body  as  a  small  depression.  In  each  case  two  secondary 
germinal  layers  take  part  in  the  coalescence — in  the  gut-wall 
the  gut-gland  layer  and  gut-fibre  layer  ;  in  the  ventral  wall 
the  skin-fibre  layer  and  skin-sense  layer. 

With  the  formation  of  the  internal  navel  and  the  closing 
of  the  alimentary  canal  is  connected  the  formation  of  two 
cavities  which  we  call  the  capital  and  the  pelvic  sections  of 
the  visceral  cavity.  As  the  embryonic  shield  lies  flat  on  the 
wall  of  the  embryonic  vesicle  at  first,  and  only  gradually 
separates  from  it,  its  fore  and  hind  ends  are  independent  in 
the  beginning  ;  on   the  other  hand,  the  middle  part  of  the 


DDRSA/.  BODY  AND   VI-.XTRAI.  BODY 


ventral  surface  is  connected  with  the  yolk-sac  by  means  o( 
tin.-  vitelline  or  umbilical  duet  (Fig.  152  m).  This  leads  to  a 
notable  curving  of  the  dorsal  surface  ;  the  head-end  bends 
downwards  towards  the  breast  and  the  tail-end  towards  the 
belly.  We  see  this  very  clearly  in  the  excellent  old  diagram- 
matic illustration  given  by  Baer  (Fig.  [52),  a  median  longi- 
tudinal section  of  the  embryo  of  the  chick  in  which  the 
dorsal  body  or  episoma  is  deeply  shaded.  The  embryo 
seems  to  be  trying  to  roll  up,  like  a  hedgehog  protecting 
itself  from  its  pursuers.     This   pronounced  curve  of  the  back 


Fig.  152.  -Median  longitudinal  section  of  the  embryo  of  a  chick  (fifth 
day  of  incubation),  seen  from  the  right  side  (head  to  the  right,  tail  to  the  left). 
Dorsal  body  (episoma)  dark,  with  convex  outline,  <l  gut,  o  mouth,  a  amis. 
/  lungs,  //  liver,  g  mesentery,  v  auricle  of  the  heart,  k  ventricle  of  thi 
h  arch  of  the  arteries,  /aorta,  c  yelk-sac,  m  vitelline  (yelk)  duct,  u  allantois, 
r   pedicle   (-.talk)   of  the    allantois,    n   amnion,    70  amniotic    cavity   (amniocoel), 

s  serous  membr  Baer.) 

is  due  to  the  more  rapid  growth  of  the  convex  dorsal  surface, 
and  is  directly  connected  with  the  severance  of  the  embryo 
from  the  yelk-sac.  At  the  head  there  is  no  division  of  skin- 
fibre  layer  from  gut-fibre  layer,  as  there  is  in  the  trunk,  but 
the  two  remain  joined,  and  are  called  the  "  head-plates."  As 
these  head-plates  release  themselves  at  an  early  stage  from 
the  surface  of  the  germinative  area,  and  grow,  first  downwards 
towards  the  surface  of  the  embryonic  vesicle  and  then  back- 
wards towards  its  passage  into  the  alimentary  groove,  a 
small  ca\ity   is  formed  within  the  head-part     this  represents 


3*8 


DORS.  \L  BODY  A  XD    I  'EXTRA  L  BOD  1 _ 


the  foremost  and  blindly  closed  part  of  the  gut.  It  is  the 
small  "head-cavity  of  the  gut"  (Fig.  153,  above  d) ;  its 
opening  in  the  middle  gut  is  called  the  "  fore  entrance  of  the 
gut"  (Fig.  153  at  d).  It  corresponds  to  the  branchial  gut  of 
the  amphioxus,  which  nearly  occupies  the  fore  half  of  the 
body.  The  tail-end  bends  for- 
ward to  the  ventral  side  in  just 
the  same  way ;  this  causes  the 
ventral  wall  to  enclose  a 
similar  small  cavity,  the  pelvic 
cavity  of  the  gut,  the  hind  end 
of  which  is  closed.  Its  open- 
ing in  the  middle  gut  is  called 
the  "  hind  entrance  of  the  gut." 
As  a  result  of  these  pro- 
cesses the  embryo  attains  a 
shape  that  may  be  compared 
to  a  wooden  shoe,  or,  better 
still,  to  an  overturned  canoe. 
Imagine  a  canoe  or  boat  with 
both  ends  rounded  and  a  small 
covering  before  and  behind  ; 
if  this  canoe  is  turned  upside 
down,  so  that  the  curved  keel 
is  uppermost,  we  have  a  fair 
picture  of  the  canoe-shaped 
embryo   (Fig.    152).     The    up. 


Fig.  153.— Longitudinal  section 
Of  the  f'OPe  half  of  a  chick-embryo 

at  the  end  of  the  first  day  of  incu- 
bation (seen  from  the  left  side). 
k  head-plates,  ch  chorda.     Above  it 

is  the  wind  fore-end  of  the  ventral    turned  convex  keel  corresponds 

tube    ( in )  ;    below     it     the    capital  .,,,,.  r     ,       ,         , 

to  the  middle  line  of  the  back  ; 


cavity  of  the  guti  d  gut-gland  layer, 
df  gut-fibre  layer,  //  horn-plate,  hh 
cavity  of  the  heart,  ££  heart-capsule, 

ks  head-sheath,  kk  head-capsule. 
(From  Rfinak.) 


the  small  chamber  underneath 
the  fore-deck  represents  the 
capital  cavity,  and  the  small 
chamber  under  the  rear-deck  the  pelvic  chamber  of  the  gut 
(cf.  Fig.  145). 

The  embryo  now,  as  it  were,  presses  into  the  outer  surface 
of  the  embryonic  vesicle  with  its  free  ends,  while  it  moves 
away  from  it  with  its  middle  part.  As  a  result  of  this  change 
the   yelk-sac   becomes    henceforth    only   a    pouch-like   outer 


POlCS.l/.  HOnV  AND   VENTRAL  BODY 


appendage  at  the  middle  of  the  ventral  wall.  The  ventral 
appendage,  growing  smaller  and  smaller,  is  afterwards  ealled 
the  umbilical  (navel)  vesicle.  (Cf.  Figs.  146,  147  us;  Fig.  151 
and  Plate  VII.,  Figs.  14,  15.)  The  cavity  of  the  yelk-sac  or 
umbilical  vesicle  communicates  with  the  corresponding 
visceral  cavity  by  a  wide  opening,  which  gradually  contracts 
into  a  narrow  and  long  canal,  the  vitelline  (yelk)  duct 
(ductus  vitellinus,  Fig.  152  in).  Hence,  if  we  were  to 
imagine  ourselves  in  the  cavity  of  the  yelk-sac,  we  could 
get  from  it  through  the  yelk-duct  into  the  middle  and  still 
wide  open  part  of  the  alimentary  canal.  If  we  were  to  go 
forward  from  there  into  the  head-part  of  the  embryo,  we 
should  reach  the  capital  cavity  of  the  gut,  the  fore-end  of 
which  is  closed  up.  Hence  the  first  structure  of  the  alimen- 
tary canal  consists  now  of  three  different  sections:  (1)  The 
capital  cavity,  which  opens  behind  (through  the  fore-opening 
of  the  gut)  into  the  middle  gut;  (2)  the  middle  cavity,  which 
opens  below  (through  the  vitelline  duct)  into  the  yelk-sac  ; 
and  (3)  the  pelvic  cavity,  which  opens  outwards  (by  the  hind 
aperture  of  the  gut)  into  the  middle  gut. 

The  reader  will  ask :  "  Where  are  the  mouth  and  the 
anus?  "  These  are  not  at  first  present  in  the  embryo.  The 
whole  of  the  primitive  gut-cavity  is  completely  closed,  and  is 
merely  connected  in  the  middle  by  the  vitelline  duct  with  the 
equally  closed  cavity  of  the  embryonic  vesicle  (Fig.  145). 
The  two  later  apertures  of  the  alimentary  canal — the  anus 
and  the  mouth — are  secondary  constructions,  formed  from 
the  outer  skin.  In  the  horn-plate,  at  the  spot  where  the 
mouth  is  found  subsequently,  a  pit-like  depression  is  formed, 
and  this  grows  deeper  and  deeper,  pushing  towards  the 
blind  fore-end  of  the  capital  cavity  ;  this  is  the  mouth-pit. 
In  the  same  way,  at  the  spot  in  the  outer  skin  where  the  anus 
is  afterwards  situated  a  pit-shaped  depression  appears,  grows 
deeper  and  deeper,  and  approaches  the  blind  hind-end  of  the 
pelvic  cavity  ;  this  is  the  anus-pit.  In  the  end  these  pits 
touch  with  their  deepest  and  innermost  points  the  two  blind 
ends  oi  the  primitive  alimentary  canal,  so  that  they  are  now 
only  separated  from   them    by   thin   membranous  partitions. 


DORSAL  BODY  AXD    VEXTRAL  BODY 


This  membrane  finally  disappears,  and  henceforth  the 
alimentary  canal  opens  in  front  at  the  mouth  and  in  the 
rear  by  the  anus  (Figs.  146,  152).  Hence  at  first,  if  we 
penetrate  into  these  pits  from  without,  we  find  a  partition 
cutting  them  off  from  the  cavity  of  the  alimentary  canal, 
which  gradually  disappears.  The  formation  of  mouth  and 
anus  is  secondary  in  all  the  vertebrates. 

The  remainder  of  the  embryonic  vesicle,  which  we  have 
called  the  umbilical  vesicle  or  yelk-sac,  becomes  smaller  and 

Mesoderm 


Bend  of 
skull  ' 


Umbilical 
cord 


Terminal 
gut 


Rudimentary 


kidneys 

Fig.   154.— Longitudinal  section  of  a  human  embryo  of  the  fourth 

week,  five  mm.  long-,  magnified  fifteen  times.     (From  KoUmann.) 

smaller,  and  at  last  hangs  out  like  a  little  pouch  from  the 
middle  of  the  gut  by  a  thin  pedicle,  the  vitelline  duct  (Fig. 
147  ds).  This  vitelline  duct  has  no  permanent  importance  ; 
it  is  afterwards,  like  the  yelk-sac,  completely  atrophied  and 
used  up.  Its  contents  are  taken  into  the  gut,  while  the  duct 
itself  grows.  The  point  at  which  it  connects  with  the  gut  is 
the  visceral  navel.  Here  in  the  end  the  alimentary  canal 
closes  up  altogether.  (Cf.  Chapter  XV.  and  Fig.  154  ;  also 
Plate  VII.,  Figs.  14,  15.) 

During    these    important   processes,  which    lead    to    the 


PDA'S.!/.  BODY  AND  VENTRAL  BODY 


formation  of  the  intestinal  wall  and  ventral  wall,  we  find  a 
number  of  other  interesting  changes  taking  place  in  the 
embryonic  shield  of  theamniotes.  These  relate  chiefly  to  the 
prerenal  ducts  and  the  first  blood-vessels.  The  prorenal 
(primitive  kidney)  ducts,  which  at  first  lie  quite  flat  under  the 
horn-plate  or  epiderm  (Fig.  141  ling),  soon  back  towards  each 


1*7^ ml 

If   M 

0  tS 


Fig.  155.— Transverse  section  of 
a  human  embryo  of  fourteen  days. 
mr  medullary  tube,  ch  chorda,  vtt 
umbilical  vein,  ml  myotome,  mf>  middle 
plate,  ug  prorenal  duct,  lh  body-cavity, 
<•  ectoderm,  Ith  ventral  skin,  /;_/"  skin- 
fibre  layer,  df  gut-fibre  layer.  (  From 
Kallmann. ) 

Fig.  i.,().— Transverse  section  of  a  shark-embryo  (or young  selachius). 
mr  medullary  tube,  ch  chorda,  u  aorta,  d  gut,  vp  principal  (or  subintestinal) 

vein,  ml  myotome,  mm  muscular  mass  o(  the  provertebra,  m/>  middle  plate, 
ug  prorenal  duet,  lh  body-cavity,  e  ectoderm  of  the  rudimentary  extremities, 
ma  mesenchymic  cells,  a  point  where  the  myotome  and  nephrotome  separate. 
I  From  II.  E.  Ziegler. ) 

other  in  consequence  of  special  growth  movements  (Figs. 
14S  150  ung).  The  direction  they  take  in  this  corresponds 
to  the  limit  between  the  dorsal  body  and  the  ventral  body 
(cf.  Figs.  155  and  156).  While  they  advance  between  the 
stem-zone  and  parietal  zone  of  the  embryonic  shield  of  the 
amniote,  they  depart  more  and  more  from  their  point  of 
origin,  and  approach  the  gut-gland  layer.  In  the  end  they 
lie   deep  in   the    interior,   on  either   side  of  the    mesentery, 


DORSAL  BODY  AND   VENTRAL  BODY 


underneath  the  chorda  (Fig.  150  ung).  At  the  same  time 
the  two  primitive  aortas  change  their  position  (cf.  Figs.  141- 
150  ao);  they  travel  inwards  underneath  the  chorda,  and 
there  coalesce  at  last  to  form  a  single  secondary  aorta,  which  is 
found  under  the  rudimentary  vertebral  column  (Fig.  150  ao). 


Fig.  157.— Transverse  section  of  a  duek-embryo  with  twenty-four 

primitive  segments.  (From  Balfour.)  From  a  dorsal  lateral  joint  of  the 
medullary  tube  (spc)  the  spinal  knots  (spg)  grow  out  between  it  and  the  horn- 
plate,  ch  chorda,  ao  double  aorta,  liy  gut-gland  layer,  sp  gut-fibre  layer,  with 
blood-vessels  in  section,  ms  muscle  plate,  in  the  dorsal  wall  of  the  myoccel 
(episomite).  Below  the  cardinal  vein  (cav)  is  the  prerenal  duct  (tvd)  and  a 
segmental  prerenal  canal  fs/J.  The  skin-fibre  layer  of  the  body-wall  (so)  is 
continued  in  the  amniotic  fold  (am).  Between  the  four  secondary  germinal 
layers  and  the  structures  formed  from  them  there  is  formed  embryonic 
connective  matter  with  stellate  cells  and  vascular  structures.  [Her twig's 
"  mesenchym.") 

The  cardinal  veins,  the  first  venous  blood-vessels,  also 
back  towards  each  other,  and  eventually  unite  immediately 
above  the  rudimentary  kidneys  (Figs.  150  vc,  157  cav).  In 
the  same  spot,  at  the  inner  side  of  the  fore-kidneys,  we  soon 
see  the  first  trace  of  the  sexual  organs.  The  most  important 
part  of  this  apparatus  (apart  from  all  its  appendages)  is  the 


/HMW.l/.  /;<)/>)■  .l\/>  VENTRAL  BODY 


ovarj  in  the  female  and  the  testicle  in  the  male.  Both 
develop  from  a  small  part  of  the  ceelous  epithelium,  the  cell- 
covering  o(  the  body-cavity,  at  the  spot  where  the  skin-fibre 
layer  and  gut-fibre  layer  touch.  The  connection  of  this 
embryonic  gland  with  the  prorenal  ducts,  which  lie  close 
to  it  and  assume  most  important  relations  to  it,  is  only 
secondary.     (Cf.  Chapter  XXIX.  and  Plate  VI.,  Figs.  4-8.) 


THIRTEENTH  TABLE 
SYNOPSIS  OF  THE  COMPOSITION  OF  THE 
VERTEBRATE-BODY  FROM  DORSAL  AND  VEN- 
TRAL BODY,  HEAD-HALF  AND  TRUNK-HALF 


Dorsal  and 
Ventral  Body. 

Episoma  and 
hyposoma. 


Head  and  Trunk. 
Caput  and  truncus. 


Skull-less 
Animals. 


Skulled  Animals. 

Cra  n  iota. 


I. 

Dorsal  body. 

Episoma' 

( =  dorsal  shield 
or  notaspis  in  the 
amniote  embrj'o). 

"  Stem-zone  " 
(  =  prevertebral 

plates). 
(Animal  hemi- 
sphere of  the 
amphigastrula, 
Figs.  43-50.) 
Neural  region. 


I.   A. 

Head-hall' of  the 

dorsal  body. 

(Episoma 
capitale.) 


a.  Si  raple  pro- 
cerebral  ves- 
icles. 

b.  Three  pairs  of 
simple  organs 
of  sense. 

c.  No  rudimen- 
tary brain. 


a.  Brain  (with  five 
cerebral  ves- 
icles). 

b.  Three  pairs  of 
complex  organs 
of  sense. 

c.  Cartilaginous 
rudiment  a  ry 
brain. 


/'a.  Spinal  marrow. 

fa.  Spinal  marrow. 

I.   B. 

b.  Simple   unarti-  ' 

b.  Segmental  ver- 

Trunk-half of  the 

culated     peri-  1 

tebral  column. 

dorsal  body. 

chorda.                 1 

c.  Dorsa  1     a  n  d 

(Episoma 

c.  Dorsal     trunk- 

ventral     trunk- 

truneale.) 

muscles      with 

muscles    with- 

v      myoccel. 

V       out  myoccel. 

Horizontal  Frontal    Septum    between    Episoma    and    Hyposoma; 

Axial,  the  Endoblastie  Chorda— Lateral,  the  Eetoblastic   Prorenal 

Duets. 


11. 

Ventral  body. 

Hyposoma 

(  =  lateral  plates 

and  yelk-sac, 

besides  the 

allantois  in  the 

amniote  embryo). 

"  Parietalzone  " 

(  =  lateral  plates). 
(Vegetal  hemi- 
sphere 'of  the 
amphigastrula, 
Figs.  43-S°- ) 
Gastric  region. 


II.   A. 

Head-half  of  the 

ventral  body. 

(Hyposoma 
capitale.) 


a.  Head-wall  per- 
manent, with 
numerous  gill- 
clefts. 

b.  S  e  g  m  e  n  t  a  1 
pronephridia. 

c.  Mouth. 
Branchial  gut 
and  hypobran- 
chial  groove. 
No  floating 
bladder  or 
lungs. 

One  -  chambered 


II.   B. 

Trunk-half  of 
the  ventral  body, 

(Hyposoma' 
truneale.) 


a.  Ve  n  t  ral  wall 
(belly-  plates). 
(Parietal  layer 
of  the  hypso- 
mites). 

b.  Several  se  g- 
mental  prone- 
phridia. 

c.  Several  s  e  g- 
mental  gonades. 

d.  Stomach. 
Simple  hepatic 
tube. 

Smallintestine. 
Anus. 


a.  Head-wall  em- 
bryonal with 
from  five  to 
seven  pairs  of 
gill-clefts. 

b.  Head-kidneys 
(pronephros). 

c.  Mouth. 

Gullet      (j  a  w  - 
c  avit  y)    a  n  d 
thyreoidea. 
Floating    blad- 
der or  lungs. 

Many-chambered 

"heart. 


a.  Ventral  wall 
(belly-plates). 

(  Parietal  layer 
of  the  lateral 
plates. ) 

b.  A  pair  of  com- 
pact kidneys. 

c.  One      pair      of 
gonades. 

d.  Stomach. 
Compact  liver. 
Pancreas. 
Small  intestine. 
Large  intestine. 
Anus. 


I:      Embryonic  glands  (sex-glands). 

is     Gill-clefts  (gullet-clefts). 

/       Corium. 

lb      Liver  (kepar). 

lr      Wind-pipe  (trachea). 

hi     Lung  (fin/mo ). 

mil  Mammary  gland  (mamma). 

mg  Stomach  (stomachus). 

mil   Mouth-cavity. 

mp    Muscular  plate  (muscularis). 

n      Neural  or  medullary  tube. 

»,     Fore-brain  (cerebrum). 

;;..     Intermediate      brain     (sphere     of 

vision). 
//       Middle  brain. 
«4     Cerebellum. 
it-      Hind-brain. 


ALPHABETICAL  TABLE 

IN  EXPLANATION  OF  THE  LETTERS  ON 
PLATES  VI.  AND  VII. 

N.B.  The  ectoderm  (skin-sense  layer)  is  coloured  orange,  the  dorsal 
mesoderm  (in  the  episoma)  blue,  the  ventral  mesoderm  (in  the  hyposoma)  red, 
and  the  entoderm  (gut-gland  layer)  green. 

a       Anus. 

ii/i    Amniotic  cavity. 

/;/      Allantois  (urinary  sac). 

am   Amnion  (water-vesicle). 

ao     Aorta. 

on     Primitive  mouth  (prostoma). 

b      Ventral  muscles. 

bb     Breast-bone  (sternum ). 

c       Body-cavity  (eeeloma). 

c.      Chest  or  pleural  cavity  (cavitas 
file  uric  ). 

<.,      Peritoneal    cavity    (cavitas   peri- 
tonei). 
Gonocoel  (ventral  eeeloma  I. 

cli     Axial  rod  ( chorda). 

cm    Myoccel  (dorsal  eeeloma). 

en    Neurenteric  canal. 

it     Ccelom-pouches. 

cp    Ccelom    polar   cells  (cells   of  the 

primitive  mesoderm). 
ex    Seroccel  (extra-foetal  ccelom). 
cl       Alimentary  canal  ( trttchus ). 
dc     Large  intestine  (colon), 
dil   Small  intestine  (ileum). 
df    Cut-fibre  layer. 
its     V elk-sac  (umbilical  vesicle). 
du    Primitive  gut. 
<•       Ectoderm. 
em    Embryo. 

f     Womb  (uterus). 

Sexual  glands  (gonades). 

Sexual    plates  (embryonic  epithe- 
lium). 
h        Horn-plate  ( eerablnstiis ). 
lib      Bladder  (vesica  urinaria ). 
hf    Skin-fibre  layer. 
Ilk     Heart-ventricle  (ventri cuius). 
h!      Left  I  arterial  I  heart. 
hr     Right  (venous)  heart. 
hv     Heart-auricle  (atrium ). 
h:     Heart  (cur). 
i      Entoderm. 

VO      Call-bladder  (  vesica  fellea  ). 


S 


Spinal  marrow  (medulla  spinalis). 

Mouth  ( osculum ). 

Pancreas. 

Organs  of  sense. 

Dorsal  muscles. 

Ribs  ( custic ). 

Skull  (cranium ). 

Pubic  bone  ( os  pubis  ). 

Gullet  (pharynx). 

Skeleton-plate. 

Oesophagus. 

Mesentery. 

Prerenal  duct  ( ncpliroiluctus). 

Prerenal  tubes  ( prnniphridiei ). 

Prerenal  groove  (  nephrusuli  us  J. 

Primitive    segments    (prevertebral 

somites). 
Rudimentary  vein. 
Cardinal  veins. 
Vagina. 
Vertebra. 
Vertebral  arch. 
Body  of  vertebra. 
Legs  (limbs). 

Diaphragm. 


EXPLANATION   OF   PLATES   VI.  AND   VII. 

The  Plates  VI.  and  VII.  are  intended  to  give  a  partly  ontogenetic  and  partly 
phylogenetic  explanation  of  the  construction  of  the  human  body  from  the 
germinal  layers.  Plate  VI.  contains  only  diagrammatic  transverse  sections 
(through  the  saggital  and  the  transverse  axis);  Plate  VII.  contains  only 
diagrammatic  longitudinal  sections  (through  the  sagittal  and  the  long  axis), 
seen  from  the  left.  The  primary  layers  and  their  products  are  marked  by  the 
same  colours  throughout,  the  skin-sense  layer  orange  and  the  gut-gland  layer 
green.  The  mesoderm  and  its  products  are  blue  in  the  episoma,  or  dorsal 
body  ;  and  red  in  the  hyposoma,  or  ventral  body.  The  letters  have  the  same 
meaning  throughout.  In  all  the  figures  the  dorsal  surface  of  the  body  is 
upward,  and  the  ventral  surface  downward. 

Plate  VI.     DIAGRAMMATIC   TRANSVERSE   SECTIONS   OF 

VERTEBRATES. 

Fig.  i.  Transverse  section  of  the  gastrula  of  a  primitive  verte- 
brate (amphioxus,  of.  Fig.  10,  Plate  VII.,  longitudinal  section,  and  Figs.  40 
and  41 1.  The  whole  body  is  an  alimentary  canal  (d)  ;  the  wall  of  it  consists 
only  of  the  two  primary  layers. 

Fig.  2.  Transverse  section  of  the  eoelomula  of  a  primitive  verte- 
brate (amphioxus)  at  the  commencement  of  ccelomation.  The  dorsal  wall  of 
the  primitive  gut  (du)  divides  into  the  rudiments  of  the  median  chorda  (ch) 
and  the  two  ccelom-pouches(V/J.  The  neural  tube  ( 11)  begins  to  separate  from 
the  corneous  plate  (c).     (Cf.  Figs.  S2-S4. ) 

Fig.  ,3.    Transverse  section  of  the  chordula  (Figs.  S6-89).    The  axial 

chorda  (ch)  lies  between  the  dorsal  nerve-tube  (n)  and  the  ventral  gut- 
tube  (d).  The  ccelom-pouch  still  simple  in  the  left  (younger)  half  ( ct )  ;  in  the 
right  (older)  half  it  is  divided  by  the  lateral  furrow  into  a  dorsal  muscular 
pouch  (myoccel,  cm)  and  a  ventral  sexual  pouch  (gonoccel,  eg),  nip  muscle- 
plate,  gp  sexual-plate,  /  corium-plate,  /;  horn-plate  (outer  skin). 

Fig.  4.    Transverse  section  of  an  ideal  primitive  vertebrate  (pro- 

spondylus  or  vertebrcea,  p.  251).  The  ccelom-pouch  is  still  simple  in  the  left 
(younger)  half,  and  opens  outwardly  by  a  prorenal  canal  fas  J  into  the  lateral 
prorenal  groove  fur  J;  in  the  right  (older)  half  the  dorsal  part,  or  muscular 
pouch  frw J,  is  divided  from  the  ventral  part,  or  sexual  pouch  (eg)  ;  the  latter 
opens  by  a  prorenal  canal  (us)  into  the  prorenal  duct  ( u ),  which  has  sepa- 
rated from  the  the  horn-plate  (h).  The  right  and  left  body-cavities  arc  -.till 
separate.  In  the  gut-fibre  wall  we  see  the  first  blood-vessels,  the  arteries 
above  (aorta,  an)  and  veins  below  (principal  or  subintestinal  vein,  hi). 
ell  chorda,  11  medullary  tube,  d  alimentary  tube,  gp  sexual  plate,  nip  muscular 
plate,  /  corium-plate,  /;  horn-plate. 

Fig.  5.    Transverse  section  of  a  primitive  fish  embryo  (selachii). 

The  features  of  construction  are  almost  the  same  as  in  the  preceding  ;  only 

the  right  and   left    ccelom-pouches  have  united.     This  has  given  rise  to  the 

simple  body-cavity  (metaccel  or  pleuro-peritoneal  cavity).     The   skeletal   plate 

326 


VM    TRANSVERSE  SECTION 


PI.  17. 


■ 


L 


qlation  of  Nan  V.Ed 


LONGITUDINAL  SECTION 


I' I  VII 


e.n       n'P       t 


EXPLAN.  I  T/OJV  OF  /'/..  I  /'/■:  1 7. 


327 


also  (formed  from  the  middle  part  of  the  dorsal  ccelom-pouch)  is  more 
advanced,  and  forms  independent  "prevertebral  halves"  (lak).  As  in  Fig.  4, 
it  is  assumed  as  a  matter  of  hypothesis  that  the  cceloma  originally  opens 
outwards  (to  the  left  ?)  by  segmental  canals  (pronephridia),  but  afterwards  (to 
the  right  ')  the  dorsal  and  ventral  ccelom-pouches  are  quite  separate.  (Cf.  the 
section  in  Fig.  i.s(>.  1 

Fig.  t>.  Transverse  section  of  the  germinal  disk  of  an  amniote  (or 
higher  vertebrate),  with  rudiments  of  the  first  organs.  (Cf.  the  section  of  the 
chick  on  the  second  day  of  incubation,  Fig.  141. 1  Tin-  medullary  tube  (») 
and  tin-  prerenal  ducts  (u)art  separated  from  the  horn-plate (h).  At  each 
siilo  of  tin'  chorda  (eh)  the  provertebrae  ( n-r  )  and  the  lateral  plates  are 
differentiated.  Between  the  skin-fibre  layer  (hf)  and  the  gut-fibre  layer  (df) 
we  see  the  first  formation  of  t  le  body-cavity  or  cceloma  (eg);  underneath  it 
aro  the  two  primary  aortas  (no). 

Fig.  7.  Transverse  section  of  the  germinal  disk  of  the  same 
amniote,  a  little  further  advanced  than  Fig.  .;.  (Cf.  the  section  of  the  chick- 
embryo  on  the  third  day  of  incubation,  Fig.  148. 1  Medullary  tube  ( 11 )  and 
chorda  ( 'eh )  already  begin  to  be  enclosed  by  the  provertebrae  (ma).  The 
prerenal  ducts  (u)  are  already  completely  separated  from  the  horn-plate  (h ) 
bv  the  corium-plate  ( I )■  c  body-cavity,  <t«  aortas.  The  cutaneous  layer  rises 
up  round  the  embryo  in  the  shape  of  the  amniotic  fold  (am) ;  this  gives  rise  to 
a  space  between  the  amniotic  fold  and  the  wall  of  the  yelk-sac  (ilsj.  the 
pericoel  (serocoslom)  or  extra-foetal  cceloma  (ex). 

Fig.  s.    Transverse  section  of  the  pelvic  region  and  the  hind  limbs  of 

the  embryo  of  an  amniote.  I  Cf.  the  section  of  a  chick-embryo  on  the  fifth  day 
of  incubation,  in  Chapter  XIV. )  The  medullary  tube  (n)  is  already  entirely 
enclosed  by  the  two  arches  of  the  vertebra  (wb),a.n6  the  chorda  and  its  sheath 
by  the  two  halves  of  the  body  of  the  vertebra  ( :vk ).  The  corium-plate  (I) 
has  separated  completely  from  the  muscular  plate  (mp).  The  horn-plate  (h) 
is  much  thickened  at  the  point  oi  tin-  hind  legs  (*)•  The  sexual  parts  (g) 
extend  far  into  the  body-cavity  (c),  and  lie  close  to  the  prerenal  duet  (11). 
The  alimentary  tube  ( il )  is  fastened  by  a  mesentery  (I),  under  the  chief 
aorta  (no)  and  the  two  cardinal  veins  (  vc  ),  to  the  dorsal  surface  of  the  body- 
wall.  Below,  in  the  middle  of  the  ventral  wall,  we  see  the  pedicle  of  the 
allantois  (ctf). 

Fig.  o.  Transverse  section  of  the  thoracic  (chest)  cavity  in  man 
(diagrammatic).     The   medullary  tube  (»)  is   surrounded   by  the   developed 

vertebra.  From  the  vertebra  an  arched  rib  trees  to  right  and  left,  and 
Strengthens  the  breast-wall  ( rp ).  Below,  oil  the  ventral  surface,  the  breast- 
bone or  sternum  (lib)  lies  between  right  ami  left  ribs.  Externally,  over  the 
ribs  land  the  intercostal  muscles),  lies  tin-  outer  skin,  formed  of  the  corium- 
plate  (I)  atid  the  horny  plate  (h).  The  pectoral  cavity  (or  fore  part  of  the 
cceloma,  c)  is,  for  the  most  part,  occupied  by  the  lungs  (In  ).  in  which  the 
tubes  ramify  like  the  branches  of  a  tree.  All  these  open  together  into  the 
single  larynx  ( Ir ),  which  opens  at  tin'  neck  into  the  pharynx  (sr).  The 
(ao)  lies  between  the  alimentary  canal  and  tin'  vertebral  column.  Between 
the  trachea  and  the  sternum  is  tin-  heart,  divided  into  two  halves  by  a  partition. 
The  left  half  ( hi )  contains  only  arterial  and  the  right  ( lir )  only  venous  blood. 
Each  half  of  the  heart  is  divided  bv  a  valve  into  an  auricle  and  ventricle.  The 
heart  is  represented  diaijrammatically  in  lis  (phylogenetically)  original 
symmetrical  situation  (in  the  middle  of  the  ventral  sulci.  In  the  developed 
man  and  the  ape  the  heart  is  unsynunctrically  and  obliquely  placed,  the  apex 
being  drawn  to  the  left. 


32S  EXPLANATION  OF  PLATE  VII. 


Plate  VII.     DIAGRAMMATIC    LONGITUDINAL   SECTIONS   OF 
VERTEBRATES. 

(All  the  sections  on  this  Plate  are  seen  from  the  left.) 

Fig.  10.    Longitudinal    section   of  the   gastrula   of  a  primitive 

vertebrate  (amphioxus,  cf.  Fig.  I,  Plate  VI.,  transverse  section,  and  Figs.  40, 
41).  The  primitive  gut-cavity  opens  at  the  back  by  the  primitive  mouth  (an). 
The  body  consists  only  of  the  two  germinal  layers.  At  the  ventral  border  of 
the  primitive  mouth  one  of  the  two  large  polar  cells  of  the  mesoderm  can  be 
seen  (ccelom  pole-cells,  cp). 

Fig.  11.    Longitudinal  section  of  the  chordula  (Figs.  S6-S9).    The 

dorsal  medullary  tube  (  n)  is  connected  behind  with  the  alimentary  canal  (du) 
by  the  neurenteric  canal ;  the  axial  chorda  (ch)  lies  between  the  two. 

Fig.  12.    Lateral  view  of  a   primitive   vertebrate  (prospondylus, 

Figs.  101-105),  from  the  left  side.  The  axial  chorda  (ch)  divides  the  episoma 
from  the  hyposoma.  In  the  head  half  we  have  the  brain  (  nc)  above  and  the 
gill-gut  ( is)  below,  with  eight  pairs  of  gill-clefts  ;  in  the  trunk  half  the 
medullary  tube  (nr)  and  the  muscle-plates  (nip)  above  and  the  segmental 
gonades  (g)  below,  a  anus,  o  mouth,  ink  mouth-cavity,  q  sense  organs, 
hz  heart. 

Fig.  13.    Longitudinal  section  of  a  primitive  fish  ( proselachius),  a 

close  relation  of  the  actual  sharks  and  the  hypothetical  ancestors  of  man. 
(The  fins  are  omitted.)  The  medullary  tube  has  divided  into  the  five  primitive 
cerebral  vesicles  (n-^-n^)  and  the  spinal  marrow  (nr)  (cf.  Figs.  15,  16).  The 
brain  is  enclosed  by  the  skull  (s),  and  the  spinal  marrow  by  the  vertebral 
canal  (above  the  marrow  the  vertebral  arches,  wb  ;  underneath  it  the  bodies  of 
the  vertebra;,  wk;  under  these  again  the  source  of  the  ribs  is  indicated).  In 
front  a  sense-organ  ( '  q )  has  developed  from  the  horny  plate.  The  alimentary 
canal  (d)  has  divided  into  the  following  parts  :  mouth-cavity  ( mh),  gullet- 
cavity  with  eight  pairs  of  gill-clefts  (is),  floating-bladder  (=  lungs,  lu), 
oesophagus  (sr),  stomach  (mg),  liver  (lb)  with  the  gall-bladder  ( iv),  small 
intestine  (dd)  and  rectum  with  anus  (a).  Under  the  rectum  is  the  sexual 
gland  ( g );  higher  up,  the  primitive  kidneys  (us).  Under  the  gullet-cavity 
lies  the  heart,  with  auricle  (hv)  and  ventricle  (hi). 

Fig.  14.    Longitudinal   section   of    the  embryo   of    an   amniote, 

showing  the  relation  of  the  alimentary  canal  to  the  appendages.  In  the  middle 
the  long-stalked  yelk-sac  (or  umbilical  vesicle,  ds)  arises  from  the  alimentary 
canal ;  behind,  the  long-stalked  allantois  ( al)  also  proceeds  from  the  canal. 
Beneath  the  fore-gut  is  the  heart  (lis),  ah  amniotic  cavity.  The  ventral  part 
of  the  amnion  (ah)  encloses  the  pedicle  of  the  lecithom  and  the  allantois 
(umbilical  cord). 

Fig.  15.    Longitudinal  section  of  a  human  embryo  of  five  weeks 

(cf.  Fig.  14).  The  amnion,  the  placenta,  and  the  urachus  are  omitted.  The 
medullary  tube  has  divided  into  the  five  primitive  cerebral  vesicles  (n-y-n.) 
and  the  spinal  marrow  (nr,  cf.  Figs.  13  and  16).  The  brain  is  enclosed  by  the 
skull  (s)  ;  under  the  spinal  marrow  is  the  series  of  the  vertebral  bodies  (wi). 
The  alimentary  canal  has  been  differentiated  into  the  following  sections: 
gullet-cavity  with  three  pairs  of  gill-clefts  (is),  lungs  ( lit),  oesophagus  (sr), 
stomach  (mg),  Uver(lb),  small  intestine  (dd)  into  which  the  yelk-sac  ( ds) 
opens,  urinary  bladder  (hb),  and  rectum,  hs  heart.  The  remainder  of  the 
tail  is  still  clearly  seen  to  the  right  below. 


EXPLANATION  OF  PLATE  VII.  329 


Fig.  i".  Longitudinal  section  of  a  developed  human  female  body. 
All  tlu-  parts  are  fully  developed,  but  diagrammatically  reduced  and  simplified  in 
order  to  show  more  clearly  the  arrangement  and  the  relation  to  the  four  secondary 
germ-layers.  In  the  brain  the  five  original  vesicles  (Fig.  15,  n,  n  .  1  have 
separated  and  developed  in  the  manner  peculiar  to  the  higher  mammals  : 
«,  Fore-brain  or  cerebrum  (preponderating  over  and  covering  the  other  lour) ; 
«..  intermediate-brain  or-  optic  thalami  ;  n.,  middle  brain  or  corpora  quadri- 
gemina ;  n ,  hind-brain  or  cerebellum;  «.  after-brain  or  pons  Varoii,  passing 
into  the  spinal  corA(nr).  The  brain  is  enclosed  by  the  s\oM.(s),  t lie  spinal 
cord  by  the  vertebral  canal ;  above  the  cord  are  the  vertebral  arches  and 
spinal  processes  (:^bj.  beneath  it  the  bodies  of  the  vertebrae  (ink).  The 
alimentary  canal  lias  boon  divided  into  the  following  successive  sections: 
mouth-cavity,  gullet-cavity  (in  which  the  gill-clefts,  is.  were  Formerly),  wind- 
pipe ( lr )  with  lungs  (hi),  oesophagus  (sr ),  stomach  ftngj,  liver  ( lh )  with 
gall-bladder  (  iv),  pancreas  ( p),  small  intestine  ( <ld  )  and  large  intestine-  (dc), 
rectum  and  amis  (a).  The  body-cavity  or  coeloma  (c)  is  divided  by  the 
diaphragm  (:)  into  two — the  thoracic-cavit}  fej,m  which  we  have  the 
heart  ( liz )  in  front  of  the  lungs;  and  the  abdominal-cavity,  in  which  are  most 
of  the  viscera.  In  front  of  the  rectum  is  the  female  vagina  (  vg  ),  which  leads 
into  the  womb  (uterus,  f) ;  in  this  the  embryo  (indicated  by  a  small  embryonic 
vesicle,  em)  developes.  Between  the  uterus  and  the  os  pubis  ( sb )  lies  the 
bladder  (  hh  ),  the  remainder  of  the  pedicle  of  the  allantois.  The  horn-plater'/;,) 
covers  the  entire  hotly  as  the  epidermis,  and  also  lines  the  cavities  of  the 
mouth,  the  anus,  the  vagina,  and  the  womb.  The  mammary  gland  (md)a\so 
was  originally  formed  from  the  corneous  plate. 


NOTE.— The  four  colours  that  are  used  on  Plates  VI.  and  VII.  in  explaining 
human  organogenesis  only  correspond  in  purl  to  the  four  secondary  germinal 
layers.  The  skin-sense  (cutaneous  sensory)  layer  is  orange,  the  gut-gland 
(intestino-glandular)  layer  green.     On  the   other  hand,  nil  organs  are  blue  iii 

the   episoma  and   red    in    the    hyposoma — whether    they  are    products    of    the 
parietal  middle  layer  I  skin-fibre  layer)  or  the  visceral  mesoderm  (gut-fibre  layer). 


CHAPTER   XIV. 

THE   ARTICULATION    OF   THE    BODY 

Metamerism  or  articulation  of  the  body  of  the  higher  animals  :  division  into  a 
chain  of  segments  or  consecutive  parts.  Internal  articulation  of  the  verte- 
brates and  external  segmentation  ot~  the  articulates  resemble  each  other, 
but  differ  profoundly.  Beginning  of  articulation  of  the  amniotes  in  the 
middle  of  the  embryonic  shield.  Increase  of  the  somites  or  primitive 
segments  from  front  to  back.  Their  number  in  man.  Segments  of  the  head 
and  of  the  trunk.  Articulation  of  the  amphioxus.  Severance  of  the  somites 
from  the  fore-end  of  the  ccelom-pouches.  Division  of  each  primitive  segment 
into  a  dorsal  (myotome)  and  a  ventral  (gonotome)  half.  Segmentation  of 
the  craniotes :  segmental  protovertebral  plates  and  unarticulated  lateral 
plates.  Differentiation  of  the  metamera  in  the  fishes,  amphibia,  and  amniotes. 
Segmentation  of  the  episoma  and  hyposoma.  Original  metamerism  of  the 
gonades  and  nephridia.  Articulation  of  the  fore-gut  :  gill  clefts  and  arches. 
Primary  and  secondary  metamerism.  Monomeric  organs  :  heart,  lungs, 
liver,  sense-organs,  limbs.  Similarity  of  vertebrate-embryos  and  its  phylo- 
genetic  significance. 

The  vertebrate  stem,  to  which  our  race  belongs  as  one  of  the 
latest  and  most  advanced  outcomes  of  the  natural  biogenetic 
process,  is  rightly  placed  at  the  head  of  the  animal  kingdom. 
This  privilege  must  be  accorded  to  it,  not  only  because  man 
does  in  point  of  fact  soar  far  above  all  other  animals,  and  has 
been  lifted  to  the  position  of  "lord  of  creation";  but  also 
because  the  vertebrate  organism  far  surpasses  all  the  other 
animal-stems  in  size,  in  complexity  of  structure,  and  in  the 
advanced  character  of  its  functions.  From  the  point  of  view 
of  both  morphology  and  physiology,  the  vertebrate  phylum 
(stem)  outstrips  all  the  other,  or  invertebrate,  animals. 

There  is  only  one  among  the  twelve  stems  of  the  animal 
kingdom  that  can  in  many  respects  be  compared  with  the 
vertebrates,  and  reaches  an  equal,  if  not  a  greater,  importance 
in  many  points.  This  is  the  stem  of  the  articulates,  composed 
of  three  classes:  i.  The  annelids  (rain-worms,  leeches,  and 
cognate  forms)  ;  2.  The  Crustacea  (crabs  and  tortoises,  etc.)  ;. 
3.  The  tracheata  (peripatida,  myriapods,  spiders,  and  insects). 
The  phylum  of  the  articulates  is  superior  not  only  to  the 

35° 


THE  ARTICl  'I- A  TION  OF  THE  BODY  33  • 

vertebrates,  but  to  all  other  animal-stems,  in  variety  of  forms, 
number  o(  species,  size  of  individuals,  and  general  importance 
in  the  economy  of  nature. 

When  we  have  thus  declared  the  vertebrates  and  the  arti- 
culates to  be  the  most  important  and  most  advanced  of  the 
twelve  stems  of  the  animal  kingdom,  the  question  arises 
whether  this  special  position  is  accorded  to  them  on  the 
ground  of  a  peculiarity  of  organisation  that  is  common  to 
the  two.  The  answer  is  that  this  is  really  the  case  ;  it  is  the 
segmental  or  transverse  articulation,  which  we  may  briefly 
call  metamerism.  In  all  the  vertebrates  and  articulates  the 
developed  individual  consists  of  a  series  of  successive 
members  (segments  or  metamera  =  "  parts") ;  in  the  embryo 
these  are  called  primitive  segments  or  somites.  In  each  of 
these  metamera  we  have  a  certain  group  of  organs  reproduced 
in  the  same  arrangement,  so  that  we  may  regard  each  segment 
as  an  individual  unity,  or  a  special  "  individual  "  subordinated 
to  the  entire  personality. 

The  similarity  of  the  morphological  segmentation,  and  the 
consequent  physiological  advance  in  the  two  stems  of  the 
vertebrates  and  articulates,  has  led  to  the  assumption  of  a 
direct  affinity  between  them,  and  an  attempt  to  derive  the 
former  directly  from  the  latter.  The  annelids  were  supposed 
to  be  the  direct  ancestors,  not  only  of  the  Crustacea  and 
tracheata,  but  also  of  the  vertebrates.  We  shall  see  later 
(Chapter  XX.)  that  this  annelid  theory  of  the  vertebrates  is 
entirely  wrong,  and  ignores  the  most  important  differences  in 
the  organisation  of  the  two  stems.  The  internal  articulation 
of  the  vertebrates  is  just  as  profoundly  different  from  the 
external  metamerism  of  the  articulates  as  are  their  skeletal 
structure,  nervous  system,  vascular  system,  and  SO  on.  The 
metamerism  has  been  developed  in  a  totally  different  way  in 
the  two  stems.  The  unarticulated  chordula  (Figs.  86-89), 
which  we  have  recognised  as  one  of  the  chief  palingenetic 
embryonic  forms  of  the  vertebrate  group,  and  from  which  we 
have  inferred  the  existence  of  a  corresponding  ancestral  form 
for  all  the  vertebrates  and  tunicates,  is  quite  unthinkable  as 
the  stem-form  of  the  articulates. 


THE  ARTICULATION  OF  THE  BODY 


All  articulated  animals  came  originally  from  unarticulated 
ones.  This  phylogenetic  principle  is  as  firmly  established 
as  the  ontogenetic  fact  that  every  articulated  animal-form 
developes  from  an  unarticulated  embryo.  But  the  organisa- 
tion of  the  embryo  is  totally  different  in  the  two  stems.  The 
palingenetic  chordula-embryo  of  all  the  vertebrates  is  charac- 
terised by  the  dorsal  medullary  tube,  the  neurenteric  canal, 
which  passes  at  the  primitive  mouth  into  the  alimentary 
canal,  and  the  axial  chorda  between  the  two.  None  of  the 
articulates,  either  annelids  or  arthropods  (crustacea  and 
tracheata),  show  any  trace  of  this  type  of  organisation. 
Moreover,  the  development  of  the  chief  systems  of  organs 
proceeds  in  the  opposite  way  in  the  two  stems,  as  is  shown 
in  Table  XIV.  Hence  the  typical  metamerism  of  the  two 
stems  must  have  been  acquired  independently  of  each  other. 
This  is  not  at  all  surprising  ;  we  find  analogous  cases  in  the 
stalk-articulation  of  the  higher  plants  and  in  several  groups 
of  other  animal  stems — for  instance,  in  the  tape-worm  and 
gunda  (among  the  platodes),  in  the  star-fish  and  encrinite 
(among  the  echinoderms),  in  the  scyphostoma  (among  the 
cnidaria),  and  so  on. 

The  characteristic  internal  articulation  of  the  vertebrates 
and  its  importance  in  the  organisation  of  the  stem  are  best 
seen  in  the  study  of  the  skeleton.  Its  chief  and  central  part, 
the  cartilaginous  or  bony  vertebral  column,  affords  an  obvious 
instance  of  vertebrate  metamerism ;  it  consists  of  a  series  of 
homogeneous  cartilaginous  or  bony  pieces,  which  have  long 
been  known  as  vertebra?  (or  spondyli).  Each  vertebra  is 
directly  connected  with  a  special  section  of  the  muscular 
system,  the  nervous  system,  the  vascular  system,  etc.  Thus 
most  of  the  "  animal  organs  "  take  part  in  this  vertebration. 
But  we  saw,  when  we  were  considering  our  own  vertebrate 
character  (in  Chapter  XL),  that  the  same  internal  articulation 
is  also  found  in  the  lowest  primitive  vertebrates,  the  acrania, 
although  here  the  whole  skeleton  consists  merely  of  the 
simple  chorda,  and  is  not  at  all  articulated.  Hence  the 
primary  articulation  does  not  proceed  from  the  skeleton,  but 
from  the  muscular  system,   and  is   clearly  phylogenetically 


THE  ARTICULATION  OF  THE  BODY      LxD.t\x&^  ' 

determined    by  the  more  advanced  s\vimn^^-jJi>PnJ<BU.sv<te/i  "  imM 
the  primitive  chordonia-ancestors. 

1 1  is,  therefore,  wrong  to  describe  the  first  rudiments  of 
the  metamera  in  the  vertebrate  embryo  as  primitive  vertebras 
or  proloroertebrce ;  the  fact  that  they  have  been  so  called  for 
some  time  has  led  to  much  error  and  misunderstanding. 
Hence  we  shall  give  the  name  of  "somites"  or  primitive 
segments  to  these  so-called  "primitive  vertebrae."  If  the 
latter  name  is  retained  at  all,  it  should  only  be  used  of  the 
sclerotom — i.e.,  the  small  dorso-medial  part  of  the  somites 
from  which  the  later  vertebra  does  actually  develop. 

Articulation  begins  in  all  vertebrates  at  a  very  early 
embryonic  stage,  and  this  indicates  the  considerable  phylo- 
genetic  age  of  the  process.  When  the  chordula  (Figs.  86-89) 
has  completed  its  characteristic  composition,  often  even  a 
little  earlier,  we  find  in  the  amniotes,  in  the  middle  of  the 
sole-shaped  embryonic  shield,  several  pairs  of  dark  square 
spots,  symmetrically  distributed  on  both  sides  of  the  chorda 
(Figs.  134-138).  Transverse  sections  (Fig.  141  uw)  show 
that  they  belong  to  the  stem-zone  (episoma)  of  the  mesoderm, 
and  are  separated  from  the  parietal  zone  (hyposoma)  by  the 
lateral  folds;  in  section  they  are  still  quadrangular,  almost 
square,  so  that  they  look  something  like  dice.  These  pairs 
of  "  cubes  "  of  the  median  mesoderm  are  the  first  traces  of  the 
primitive  segments  or  somites,  the  so-called  "  protovertebras  " 
( Figs.  158-160  uw). 

Among  the  mammals  the  embryos  of  the  marsupials  have 
three  pairs  of  somites  (Fig.  134)  after  sixty  hours,  and  eight 
pairs  after  seventy-two  hours  (Fig.  138).  They  develop  more 
slowly  in  the  embryo  of  the  hare;  this  has  three  somites  on 
the  eighth  day  (Fig.  135),  and  eight  somites  a  day  later 
(Fig.  [37).  In  the  incubated  hen's  egg  the  first  somites 
make  their  appearance  thirty  hours  after  incubation  begins 
(Fig.  158).  At  the  end  of  the  second  day  the  number  has 
risen  to  sixteen  or  eighteen  (Fig.  160).  The  articulation  of  the 
mesodermic  stem-zone,  to  which  the  somites  owe  their  origin, 
thus  proceeds  briskly  from  front  to  rear,  new  transverse  con- 
strictions o(  the  "  protovertebral  plates  "  forming  continuously 


THE  ARTICULATION  OF  THE  BODY 


and  successively.  The  first  segment,  which  is  almost  half- 
way down  in  the  embryonic  shield  of  the  amniote,  is  the 
foremost  of  all;  from  this  first  somite  is  formed  the  first 
cervical  vertebra  with  its  muscles  and  skeletal  parts.  It 
follows  from  this,  firstly,  that  the  multiplication  of  the 
primitive  segments  proceeds  backwards  from  the  front,  with  a 


Wi  Figs.  158-160.— Sole-shaped  embryonic  disk  of  the  chick,  in  three 

successive  stages  of  development,  looked  at  from  the  dorsal  surface,  magnified 
about  twenty  times,  somewhat  diagrammatic.  Fig".  158  with  six  pairs  of 
somites.  Brain  a  simple  vesicle  (lib).  Medullary  furrow  still  wide  open  from 
x  ;  greatly  widened  at  c.  mp  medullary  plates,"  sp  lateral  plates,  y  limit  of 
gullet-cavity  ( sh )  and  fore-gut  (  vd ).  Fig.  159  with  ten  pairs  of  somites. 
Brain  divided  into  three  vesicles  :  v  fore-brain,  m  middle-brain,  h  hind-brain, 
r  heart,  dv  yelk-veins.  Medullary  furrow  still  wide  open  behind  (z),  mp 
medullary  plates.  Fig.  160  with  sixteen  pairs  of  somites.  Brain  divided  into 
five  vesicles  :  v  fore-brain,  c  intermediate-brain,  m  middle-brain,  /;  hind-brain, 
ii  after-brain,  a  optic  vesicles,  g  auditory  vesicles,  c  heart,  dv  yelk-veins,  mp 
medullary  plate,  uw  primitive  vertebra. 


THE  ARTICULATION  OF  THE  BODY 


constant  lengthening  of  the  hinder  end  of  the  body;  and, 
secondly,  that  at  the  beginning  of  segmentation  nearly  the 
whole  o(  the  anterior  half  of  the  sole-shaped  embryonic 
shield  of  the  amniote  belongs  to  the  later  head,  while  the 
whole  o\  the  rest  of  the  body  is  formed  from  its  hinder  half. 
We  are  reminded  that  in  the  amphioxus  (and  in  our  hypo- 
thetic  primitive  vertebrate,  Figs.  101-105)  nearly  the  whole 
o(  the  fore  half  corresponds  to  the  head,  and  the  hind  half  to 
the  trunk. 

The  mesoderm  of  the  amniote  head  developes  from  the 
undivided  "  head-plates,"  which  are  clearly  distinguished 
from  the  protovertebral  plates  of  the  trunk  by  the  absence  of 
articulation.  But  we  shall  sec  that  this  simplicity  of  the 
head-plates  is  not  original,  but  cenogenetic.  In  the  lower 
vertebrates  even  the  head-part  seems  to  be  clearly  articulated, 
and  composed  of  at  least  nine  somites  ;  and  in  the  embryo  of 
certain  palingenetic  fishes  as  many  as  twelve  to  fourteen  head- 
segments  have  recently  been  found.  But  in  the  higher 
vertebrates  these  head-somites  (like  head-metamera  of  the 
higher  articulates)  fuse  together  at  such  an  early  stage  that 
it  took  the  acute  observations  of  Gegenbaur  (1872)  to  prove 
them  by  comparative  anatomic  methods.  The  proof  was 
afterwards  confirmed  by  others  with  the  aid  of  comparative 
ontogeny.  We  shall  return  to  the  point  in  discussing  the 
theory  of  the  skull  in  Chapter  XXVI. 

The  number  of  the  metamera,  and  of  the  embryonic 
somites  or  primitive  segments  from  which  they  develop, 
varies  considerably  in  the  vertebrates,  according  as  the  hind 
part  of  the  body  is  short  or  is  lengthened  by  a  tail.  In  the 
developed  man  the  trunk  (including  the  rudimentary  tail) 
consists  of  thirty-three  metamera,  the  solid  centre  of  which  is 
formed  by  that  number  of  vertebra?  in  the  vertebral  column 
(seven  cervical,  twelve  dorsal,  five  lumbar,  five  sacral,  and 
four  caudal).  To  these  we  must  add  at  least  nine  head- 
vertebras,  which  originally  (in  all  the  craniota)  constitute  the 
skull.  Thus  the  total  number  of  the  primitive  segments  of 
the  human  body  is  raised  to  at  least  forty-two;  it  would  reach 
forty-five  to  forty-eight  if  (according  to  recent  investigations) 


336 


THE  ARTICULATIOX  OF  THE  BODY 


the  number  of  the  original  segments  of  the  skull  is  put  at 
twelve  to  fifteen.  In  the  tailless  or  anthropoid  apes  the  number 
of  metamera  is  the  same  as  in  man,  and  only  differs  by  one 
or  two;  but  it  is  much  larger  in  the  long-tailed  apes  and 
most  of  the  other  mammals.  In  long  serpents  and  fishes  it 
reaches  several  hundred  (sometimes  400). 

In  order  to  understand  properly  the  real  nature  and  origin 
of  articulation  in  the  human  body  and  that  of  the  higher 
vertebrates,  it  is  necessary  to  compare  it  with  that  of  the 
lower  vertebrates,  and  bear  in 
mind  always  the  phylogenetic 
connection  of  all  the  members 
of  the  stem.  In  this  the 
palingenetic  development  ot 
the  invaluable  amphioxus  once 
more  furnishes  the  key  to  the 
complex  and  cenogenetically 
modified  embryonic  processes 
of  the  craniota.  Here,  too,  it 
is  the  masterly  studies  of  Hat- 
schek  (of  Vienna)  that  put  most 
clearly  before  us  these  remark- 
able features  of  the  lowest 
vertebrate,  discovered  by 
Kowalevsky  thirty-six  years 
ago.  The  articulation  of  the 
amphioxus  begins  at  an  early 
stage  —  earlier  than  in  the 
craniotes.  The  two  ccelom-pouches  have  hardly  grown 
out  of  the  primitive  gut  (Fig.  161  c)  when  the  blind  fore 
part  of  it  (farthest  away  from  the  primitive  mouth,  it) 
begins  to  separate  by  a  tranverse  fold  (s):  this  is  the 
first  primitive  segment.  Immediately  afterwards  the  hind 
part  of  the  ccelom-pouches  begins  to  divide  into  a  series  of 
pieces  by  new  transverse  folds  (Fig.  162).  The  transverse 
constrictions  of  the  ccelom-pouches  lie  in  a  plane  vertical  to 
the  long  axis,  and  begin  at  their  dorsal  side  (Fig.  163). 
Proceeding    downwards    from    there,    they   cut    each    other 


Fig.  161.— Embryo  of  the  am- 
phioxus, sixteen  hours  old,  seen 

from  the  back.  (From  Hatschck.) 
(/primitive  gut,  u  primitive  mouth, 
p  polar  cells  .of  the  mesoderm,  c 
ccelom-pouches,  m  their  first  seg- 
ment, n  medullary  tube,  i  entoderm, 
e  ectoderm,  s  first  segment-fold. 


THE  ARTICULATION  OF  THE  /!()/)  Y 


completely  through  in  this  transverse  plane,  and  thus  break  up 

each  coelom-sac  into  a  series  of  roundish  cubic  vesicles.     The 

foremost  of  these  primitive  segments  (us  \)  is  the  first  and 

oldest;  in    Pigs.  162  and    163  there  are  already  five  formed. 

They  separate   SO   rapidly,  one   behind  the   other,  that   eight 

pairs  are  formed   within  twenty-four  hours  of  the  beginning 

o(  development,  and  seventeen  pairs  twenty-four  hours  later. 

The     number     in- 

,  ii  ,11,      us,   us*  „  us*  mi  en 

creases  as  the  em-  ■    ,  ...  , 

and  ^^^^naB^^^BBIIB^^ 

extends  back- 
wards, and  new 
cells  are  formed 
constantly  (at  the 
primitive  mouth) 
from  the  two  pri- 
mitive mesoderm  ic 
cells  (Figs.  164- 
166). 

This  typical 
articulation  of  the 
two  ccelom-sacs 
begins  very  early 
in  the  lancelet, 
before  they  are  yet 
severed  from  the 
primitive  gut,  so 
that  at  first  each 
segment  -  cavity 
(us J  still  com- 
municates by  a 
narrow  opening  with  the  gut,  like  an  intestinal  gland. 
But  this  opening  soon  closes  by  complete  severance,  pro- 
ceeding regularly  backwards.  The  closed  vesicular  somites 
then  extend  more,  so  that  their  upper  half  grows  upwards 
like  a  fold  between  the  ectoderm  (ak )  and  neural  tubef//), 
and  the  lower  half  between  the  ectoderm  and  alimentary 
canal  (ah;  Fig.   167  c,    left  half  of  the  figure).      Afterwards 


V  Fig.  163.  // 

Figs,  to.'  and  163.—  Embryo  of  the  amphioxus, 
twenty  hours  old,  with  five  somites.  Fig.  162 
left  view,  Fig-.  163  right  view.  (From  Hatschek.) 
V  torf  end,  //  hind  end,  ut,  mi,  ik  outer,  middle, 
and  inner  germinal  layers;  ,11,  alimentary  canal, 
n  neural  tube,  en  canalis  neurentericus,  „sh  ccelom- 
pouches  (or  primitive  segment  cavities),  us  first  (and 
foremost)  primitive  segment. 


33S 


THE  ARTICULATION  OF  THE  BODY 


the  two  halves  completely  separate,  a  lateral  longitudinal 
fold  cutting  between  them  (mk,  right  half  of  Fig.  167). 
The  dorsal  segments  ( sd )  provide  the  muscles  of  the 
trunk  the  whole  length  of  the  body  (Fig.  165) :  this  cavity 
afterwards    disappears.       On    the    other    hand,    the    ventral 


Fig.  164. 


Fk 


Fig.  166. 


Figs.  164-166.  —Embryo  of  the  amphioxus,  twenty-four  hours  old. 

With  eight  Somites.  (From  Hatschct. )  Figs.  164  and  165  lateral  view  (from 
left).  Fig.  166  seen  from  back.  In  Fig.  164  only  the  outlines  of  the  eight 
primitive  segments  are  indicated,  in  Fig.  165  their  cavities  and  muscular  walls, 
("fore  end,  H  hind  end,  d  gut,  du  under  and  dd  upper  wall  of  the  g-ut,  ne 
canalis  neurentericus,  uv  ventral,  nd  dorsal  wall  of  the  neural  tube,  ///>  neuro- 
porus,  dv  fore  pouch  of  the  gut,  ch  chorda,  mf  mesodermic  fold,  pin  polar 
cells  of  the  mesoderm  ( >>is  ),  e  ectoderm. 

somites  give  rise,  from  their  uppermost  section,  to  the  prone- 
phridia  or  prerenal  canals,  and  from  the  lower  to  the 
segmental  rudiments  of  the  sexual  glands  or  gonades.  The 
partitions  of  the  muscular  dorsal  pieces  (myotomes )  remain, 
and  determine  the  permanent  articulation  of  the  vertebrate 
organism.  But  the  partitions  of  the  extensive  ventral  pieces 
( gonotomes )  become    thinner,  and   afterwards   disappear    in 


THE  ARTICULATION  OF  THE  BODY  339 

part,  so  that  their  cavities  run  together  to  form  the  metacoel, 
or  the  simple  permanent  body-cavity. 

The  articulation  proceeds  in  substantially  the  same  way  in 
the  other  vertebrates,  the  craniota,  starting  from  the  ecelom- 
pouches.  But  whereas  in  the  former  case  there  is  first  a 
transverse  division  oi  the  coelom-sacs  (by  vertical  folds)  and 
then  the  dOrso-ventral  division,  the  procedure  is  reversed  in 
the  craniota;  in  their  case  each  of  the  long  ccelom-pouches 
first  divides  into  a  dorsal  (primitive  segment  plates)  and  a 
ventral  (lateral  plates)  section  by  a  lateral  longitudinal  fold. 
Only  the  former  are  then  broken  up 
into  primitive  segments  by  the  sub- 
sequent vertical  folds;  while  the 
latter  (segmented  for  a  time  in  the 
amphioxus)  remain  undivided,  and, 
by  the  divergence  of  their  parietal 
and  visceral  plates,  form  a  body- 
cavity  that   is  unified   from   the  first. 

In  this  case,  again,  it  is  clear  that  fig.  167. —Transverse 
„,.  _„_♦  „^T., ..  1  .1,,  1%.,,,,.,  ,r  .1, ,  section  of  the  middle  of  an 
we    must   regard  the  features  of  the     amphioxus -embryo  with 

younger  craniota  as  cenogeneticallv     eleven      primitive      seg- 

°  °  ■        ments.   ( 1-  rom  Hatschck.  |  To 

modified  processes  that  can  be  traced     the  left  the  segment  is  -.tin 

, .  ,,  ,  ,  1  •  simple,    to    the   ritrht   alreadv 

palmgenetically  to  the  older  acrania.     divided    by  the   lateral    fold 
We    have    an   interesting    inter-     (^ into  dorsal  and  ventral 

B  halves.        ak,     ink,     ik    outer, 

mediate  stage    between  the   acrania     middle,  and   inner  germinal 

.  layers,     «     neural    tube,    ch 

and    the  fishes    in   these   and   many     chorda,  dh  alimentary  canal, 

^t  .         •         .«  *  sd  dorsal    somite,  sv  ventral 

other    respects     in    the    cyclostoma     somite>  c coeloma; 
(myxinoides  and  petromyzontes,  cf. 

Chapter  XXL).  In  particular,  the  development  of  their 
muscular  segments  (from  the  dorsal  somites)  is  nearer  to 
that  oi  the  amphioxus  than  of  the  other  vertebrates  (the 
gnathostoma).  This  is  connected  with  the  fact  that  the 
cyclostoma,  like  the  acrania,  have  no  vertebral  column, 
and  that  the  articulation  of  the  body  is  very  simple  and 
primitive  in  both  groups  ;  the  formation  of  the  head, 
especially,  remains  at  a  very  low  stage,  and  there  arc- 
no  pairs  of  limbs.  These  embryonic  processes  are  much 
more   complex    in    the   fishes,   with    which    begins    the    long 


THE  ARTICULATIOX  OF  THE  BODY 


series  of  gnathostome  ("jaw-mouthed")  vertebrates  with  two 
pairs  of  extremities. 

Among  the  fishes  the  selachii,  or  primitive  fishes,  yield 
the  most  important  information  on  these  and  many  other 
phylogenetic  questions  (Figs.  168,  169).  The  careful  studies 
of  Riickert,  Van  Wijhe,  H.  E.  Ziegler,  and  others,  have 
given  us  most  valuable  results.     The  products  of  the  middle 


Fig.  16S. 


Figs.  168  and  169.— Transverse  section  of  shark-embryos  (through  the 

region  of  the  kidneys).  (From  Wijhe  and  Hertwig.)  In  Fig.  16c,  the  dorsal 
segment-cavities  (h)  are  already  separated  from  the  body-cavity  ( II:  j.  but 
they  are  connected  a  little  earlier  (Fig.  16S).  nr  neural  tube,  ch  chorda,  sch 
subchordal  string,  no  aorta,  sk  skeletal  plate,  nip  muscle-plate,  cp  cutis-plate 
w  connection  of  latter  (growth-zone),  vn  primitive  kidneys,  Kg-prorenal  duct, 
uk  prerenal  canals,  lis  point  where  they  are  cut  off',  tr  prerenal  funnel,  ink 
middle  germ-layer  {mk2  parietal,  ink.,  visceral),  it  inner  germ-layer  (gut-gland 
layer). 

germinal  layer  are  partly  clear  in  these  cases  at  the  period 
when  the  dorsal  primitive  segment  cavities  (or  tnyocoels,  //) 
are  still  connected  with  the  ventral  body-cavity  (l/i :  Fig.  168). 
In  Fig.  169,  a  somewhat  older  embryo,  these  cavities  are 
separated.  The  outer  or  lateral  wall  of  the  dorsal  segment 
yields  the  cutis-plate  (cp),  the  foundation  of  the  connective 
corium.  From  its  inner  or  median  wall  are  developed  the 
muscle-plate    ( mp,   the   rudiment   of  the  trunk-muscles)  and 


THE  ARTICULATION  OF  THE  BODY 


the    skeletal    plate,    the    formative    matter   of    the   vertebral 

column  ( sk  ). 

In  the  amphibia,  also,  especially  the  water-salamander 
( triton ),  we  can  observe  very  clearly  the  articulation  of  the 
coelom-pouches  and  the  rise  of  the 
primitive  segments  from  their  dorsal 
half  (ci.  Fig.  04,  A,  B,  C).  The 
cavity  o(  the  originally  simple 
coelom-sacs  (Fig.  94  A  and  right 
half  of  H )  remains  visible  both  in 
the  dorsal  and  ventral  segments, 
even  after  the  two  have  been 
separated  by  the  lateral  fold  (Fig. 
04  C  and  left  half  oi  li ).  A  hori- 
zontal longitudinal  or  frontal  section 
of  this  salamander-embryo  (Fig.  170) 
shows     very    clearly     the     scries    of 

paii>  of  these  vesicular  dorsal  segments,  which  have  been 
cut  off  on  each  side  from  the  ventral  side-plates,  and  lie  to 
the  right  and  left  of  the  chorda. 

The  metamerism  of  the  amniotes  agrees  in  all  essential 
points  with  that  of  the  three  lowerclasses  of  vertebrates  we  have 


Fig.  170.— Frontal  (or 
horizontal  -  longitudinal) 
section  of  atriton-embryo 

with  three  pairs  ol'  primitive 
segments.  eh  chorda,  us 
primitive  segments,  ush  their 
cavity,  ak  horn  plate. 


Fig.  171.— Transverse  section  of  a  chick-embryo  of  the  second 
day  of  incubation.  (From  KSUiker.)  mr  medullary  tube,  ch  chorda,  uw 
protovertebra,  ung  prorenal  ducts,  u"  primitive  aorta,  //:.'//  prevertebral  cavity, 
mi  primitive  kidneys,  h  horn-plate,  o/amniotic  told,  lip  skin-fibre  layer,  <^ gut- 
fibre  layer,  />  COilom,  till yelk-gland  layer. 

considered  ;  but  it  varies  considerably  in  detail,  in  conse- 
quence of  cenogenetic  disturbances  that  are  due  in  the  first 
place  (like  the  degeneration  o(  the  ccelom-pouches)  to  the  large 
development  of  the  food-yelk.  As  the  pressure  of  this  seems 
to  force  the  two  middle  layers  together  from  the  start,  and  as 


THE  ARTICULATION  OF  THE  BODY 


the  solid  structure  of  the  mesoderm  apparently  belies  the 
original  hollow  character  of  the  sacs,  the  two  sections  of  the 
mesoderm,  which  are  at  that  time  divided  by  the  lateral  fold 
— the  dorsal  segment-plates  and  ventral  side-plates — have  the 
appearance  at  first  of  solid  laminae  of  cells  (Figs.  97-100). 
And  when  the  articulation  of  the  somites  begins  in  the  sole- 
shaped  embryonic  shield,  and  a  couple  of  protovertebrae  are 


Fig.  172.— Transverse  section  of  the  embryo  of  a  chick  of  the  fourth 

day,  magnified  about  one  hundred  times.  The  protovertebrse  have  split  into 
the  outer  muscle-plate  ( mpj  and  the  inner  skeletal  plate.  The  latter  begins  to 
enclose  the  chorda  ( ch )  below  as  the  body  of  the  vertebra;  f'v/ij,  and  the 
medullar}-  tube  ( m )  above  as  the  arch  of  the  vertebrae  (ivb),  the  cavity  of  the 
medullary  tube  ( m/i J  being-  now  very  narrow.  At  «-y  the  muscular  plate 
advances  into  the  ventral  wall  ( lip  ),  hpr  corium-plate  or  dorsal  wall,  h  horny 
plate,  a  amnion,  ung  prorenal  duct,  un  prorenal  canals,  ao  primitive  artery 
(aorta),  vc  cardinal  vein,  df  gut-fibre  layer,  dd  gut-gland  layer,  dr  alimentary 
groove. 

developed  in  succession,  constantly  increasing  in  number 
towards  the  rear,  these  cube-shaped  somites  (formerly  called 
protovertebrae,  or  primitive  vertebrae)  have  the  appearance  of 
solid  dice,  made  up  of  mesodermic  cells  (Fig.  141).  Never- 
theless, there  is  for  a  time  a  ventral  cavity,  or  prevertebral 
cavity,  even  in  these  solid  "protovertebrse"  (Fig.  157  uwh). 
This  vesicular  condition  of  the  provertebra  is  of  the  greatest 


THE  ARTICULATION  OF  THE  liOltY 


phylogenetic  interesl  ;  we  must,  according  to  the  coelom 
theory,  regard  it  as  an  hereditary  reproduction  of  the 
vesicular  dorsal  somites  of  the  amphioxus  (Figs.  161-167) 
and  the  lower  vertebrates  (Figs,  168-170).  This  rudimentary 
"  prevertebral  cavity "  has  no  physiological  significance 
whatever  in  the  amniote-embryo  ;  it  soon  disappears,  being 
tilled  up  with  cells  of  the  muscular  plate. 

Another  variation  in  the  formation  of  the  segments  in  the 
amniotes  is  that  the  development  of  the  muscular  plates  from 
the  inner  (median)  wall  of  their  somites  spreads  to  the  outer 
(lateral)  wall  ;  hence  here  the  cell-stratum  of  the  "  skin-fibre 
layer,"  which  lies  directly  below  the  cutis-plate  (the  later 
corium-plate,  Fig.  169  cp),  also  takes  a  lively  part  in  the 
further  growth  of  the  muscular  plate.  It  grows  out  on  all 
sides  from  this  point,  especially  downwards  into  the  lateral 
plates  of  the  ventral  wall  (the  ventral  plates). 

The  innermost  median  part  of  the  primitive  segment 
plates,  which  lies  immediately  on  the  chorda  (Fig.  172  c/i) 
and  the  medullary  tube  f»ij,  forms  the  vertebral  column  in 
all  the  higher  vertebrates  (it  is  wanting  in  the  lowest)  ;  hence 
it  may  be  called  the  skeleton  plate.  In  each  of  the  pro- 
vertebra;  it  is  called  the  "sclerotome"  (in  opposition  to  the 
outlying  muscular  plate,  the  "  myotome").  From  the  phylo- 
genetic point  of  view  the  myotomes  are  much  older  than  the 
sclerotomes.  The  lower  or  ventral  part  of  each  sclerotome 
(the  inner  and  lower  edge  of  the  cube-shaped  provertebra) 
divides  into  two  lamina;,  which  grow-  round  the  chorda,  and 
thus  form  the  foundation  of  the  body  of  the  vertebra  (wh). 
The  upper  lamina  presses  between  the  chorda  and  the 
medullary  tube,  the  lower  between  the  chorda  and  the 
alimentary  canal  (Fig.  142  C).  As  the  laminae  of  two 
opp  site  prevertebral  pieces  unite  from  the  right  and  left, 
a  circular  sheath  is  formed  round  this  part  of  the  chorda. 
From  this  developes  the  body  oi'  a  vertebra — that  is  to  say, 
the  massive  lower  or  ventral  half  of  the  bony  ring,  which  is 
called  the  "vertebra"  proper  and  surrounds  the  medullary 
tube  (Figs.  173  173).  The  upper  or  dorsal  half  of  this  bony 
ring,  the  vertebral  arch  (Fig.   172  wb)  arises  in  just  the  same 


THE  ARTICULATIOX  OF  THE  BODY 


way  from  the  upper  part  of  the  skeletal  plate,  and  therefore 
from  the  inner  and  upper  edge  of  the  cube-shaped  primitive 
vertebra.  As  the  median  upper  edges  of  two  opposing 
somites  grow  together  over  the  medullary  tube  from  right 
and  left,  the  vertebral  arch  becomes  closed. 

The  whole  of  the  secondary  vertebra,  which  is  thus 
formed  from  the  union  of  the  skeletal  plates  of  two  pro- 
vertebral  pieces  and  encloses  a  part  of  the  chorda  in  its 
body,  consists  at  first  of  a  rather  soft  mass  of  cells  ;  this 
afterwards  passes  into  a  firmer,  second,  cartilaginous  stage, 
and  finally  into  a  third,  permanent,  bony  stage.  These  three 
stages  can  generally  be  distinguished  in  the  greater  part  of 
the  skeleton  of  the  higher  vertebrates  ;  at  first  most  parts  of 


V    - 

h'lG. 

'73- 

Fig. 

l73- 

Fig. 

174- 

Fig. 

■75- 

Fig.  174.  Fig, 

-The  third  cervical  vertebra  (human). 
-The  sixth  dorsal  vertebra  (human). 
-The  second  lumbar  vertebra  (human). 

the  skeleton  are  softer,  tender,  and  membranous  ;  they  then 
become  cartilaginous  in  the  course  of  their  development,  and 
finally  ossify. 

At  the  head  part  of  the  embryo  in  the  amniotes  there  is 
not  generally  a  cleavage  of  the  middle  germinal  layer  into 
provertebral  and  lateral  plates,  but  the  dorsal  and  ventral 
somites  are  blended  from  the  first,  and  form  what  are 
called  "the  head-plates"  (Fig.  153  k).  From  these  are  formed 
the  skull,  the  bony  case  of  the  brain,  and  the  muscles  and 
corium  of  the  body.  The  skull  developes  in  the  manner  of 
the  membranous  vertebral  column.  The  right  and  left 
halves  of  the  head  curve  over  the  cerebral  vesicle,  enclose 
the  foremost  part  of  the  chorda  below,  and  thus  finally  form 
a  simple,  soft,  membranous  capsule  about  the  brain.  This 
is  afterwards  converted    into  a  cartilaginous   primitive  skull, 


THE  ARTICULATION  OF  THE  BODY 


such  as  we  find  permanently  in  many  o\  the  fishes.  Much 
later  this  cartilaginous  skull  becomes  the  permanent  bony 
skull  with  its  various  parts.  The  bony  skull  in  man  and  all 
the  other  amniotes  is  more  highly  differentiated  and  modified 
than  that  of  the  lower  vertebrates,  the  amphibia  and  fishes. 
But  as  the  one  has  arisen  phylogenetically  from  the  other,  we 
must  assume  that  in  the  former  no  less  than  the  latter  the 
skull  was  originally  formed  from  the  sclerotomes  of  a  number 
oi  (at  least  nine)  head-somites. 

While  the  typical  articulation  of  the  vertebrate  body  is 
always  obvious  in  the  episoma  or  dorsal  body,  and  is  clearly 
expressed  in  the  metamerism  of  the  muscular  plates  and 
vertebrae  (myotomes  and  sclerotomes),  it  is  more  latent  in  the 
hyposoma  or  ventral  body.  Nevertheless,  these  ventral  hypo- 
somites  of  the  vegetal  half  of  the  body  are  not  less  important 
than  the  episomites  of  the  animal  half.  The  segmentation 
in  the  ventral  cavity  affects  the  following  principal  systems  of 
organs  :  i.  The  gonades  or  sex-glands  (gonotomes) ;  2.  The 
nephridia  or  kidneys  (nephrotomes)  ;  and  3.  The  head-gut 
with  its  metamerous  gill-clefts  (branchiotomes).  (Plate  VII., 
Fig.  12.) 

The  metamerism  of  the  hyposoma  is  less  conspicuous 
because  in  all  the  craniotes  the  gonocoels — the  cavities  of 
the  ventral  segments,  in  the  walls  of  which  the  sexual 
products  are  developed — have  long  since  coalesced,  and 
formed  a  single  large  body-cavity,  owing  to  the  disappear- 
ance of  the  partition.  This  cenogenetic  process  is  so  old 
that  the  metaccel  in  the  lateral  plates  of  the  craniota  has 
everywhere  the  appearance  from  the  first  of  a  simple  unseg- 
mented  cavity,  and  that  the  rudiment  of  the  gonades  also  is 
almost  always  unsegmented.  It  is  the  more  interesting  to 
learn  that,  according  to  the  important  discovery  of  Riickert, 
this  sexual  structure  is  at  first  segmental  even  in  the  actual 
selachii,  and  the  several  gonotomes  only  blend  into  a  simple 
sexual  gland  on  either  side  secondarily. 

Amphioxus,  the  sole  surviving  representative  of  the 
acrania,  once  more  yields  us  most  interesting  information;  in 
this  case   the   sexual   glands   remain   segmented    throughout 


346  THE  ARTICULATION  OF  THE  BODY 

life,  and  so  do  the  ventral  body-cavities.  The  sexually 
mature  lancelet  has,  on  the  right  and  left  of  the  gut,  a  series 
of  metamerous  sacs,  which  are  filled  with  ova  in  the  female 
and  sperm  in  the  male.  These  segmental  gonades*  are 
originally  nothing  else  than  the  real  gonotomes,  separate 
body-cavities,  formed  from  the  hyposomites  of  the  trunk. 
The  reason  why  they  have  hitherto  generally  been  misunder- 
stood, and  the  amphioxus  has  wrongly  been  credited  with  a 
simple  body-cavity,  is  that  the  latter  has  been  confused  with 
the  large  peribranchial  space. 

The  gonades  are  the  most  important  segmental  organs  of 
the  hyposoma,  in  the  sense  that  they  are  phylogenetically  the 
oldest.  We  find  sexual  glands  (as  pouch-like  appendages  of 
the  gastro-canal  system)  in  most  of  the  coelenteria,  even  in  the 
cnidaria  (medusa;),  which  have  no  nephridia.  The  latter 
appear  first  (as  a  pair  of  prorenal  canals  or  excretory  tubes)  in 
the  platodes  (turbellaria),  and  have  probably  been  inherited 
from  these  by  the  articulates  (annelids)  on  the  one  hand 
and  the  unarticulated  prochordonia  on  the  other,  and  from 
these  passed  to  the  articulated  vertebrates.  The  oldest  form 
of  the  renal  system  in  this  stem  are  the  segmental  pro- 
nephridia  or  the  metamerous  prorenal  canals,  in  the  same 
arrangement  as  Boveri  found  them  in  the  amphioxus.  They 
are  small  canals  that  lie  in  the  frontal  plane,  on  each  side  of 
the  chorda,  between  the  episoma  and  hyposoma  (Fig.  176  n); 
their  internal  funnel-shaped  opening  leads  into  the  various 
body-cavities,  their  outer  opening  is  the  lateral  furrow  of  the 
epidermis.  Originally  they  must  have  had  a  double  function, 
the  carrying  away  of  the  urine  from  the  myoccel  of  the 
episomites  and  the  release  of  the  sexual  cells  from  the 
gonoccel  of  the  hyposomites. 

The  recent  investigations  of  Riickert  and  Van  Wijhe  on 
the  mesodermic  segments  of  the  trunk  and  the  excretory 
system  of  the  selachii  show  that  these  "  primitive  fishes  "  are 
closely  related  to  the  amphioxus  in  this  further  respect.  The 
transverse  section  of  the  shark-embryo  in  Fig.  168  shows  the 
dorsal  and  ventral  halves  of  the  ccelom-pouches  still  openly 
connected.      In  the  middle  of  the  section,  in  the  frontal   axis, 


THE  ARTICULATION  OF  THE  JiODY 


the  narrow  myoccel  (or  cleft-like  "  muscular  cavity  "  of  the 
dorsal  segment)  passes  by  a  narrow  connecting  channel  (vb ) 
directly  into   the   wide   gonoccel  (Hi)  or  the  body-cavity  of 

the  ventral  segment,  from  the  epithelium  o(  which  sexual 
cells  develop.  The  narrow  connecting  channel  (vb)  becomes 
the  pronephridium,  or  prerenal  canal,  which  carries  away  the 
excretory  products  of  both  body-cavities  (the  urine  of  the 
dorsal  muscular  cavity  and  the  sexual  cells  of  the  ventral 
sexual  cavity).  Afterwards  (Fig.  169)  the  two  cavities  are 
divided  by  a  partition.  Then 
the  inner  opening  of  the 
renal  canal  only  leads  into 
the  lower  ventral  cavity. 
The  outer  opening  was  in 
the  surface  of  the  outer  skin, 
probably  in  the  lateral  furrow 
of  the  epidermis,  from  which 
the  prerenal  duct  developes 
in  the  craniotes  by  constric- 
tion (Fig.  171  ting).  In  the 
amphioxus,  as  Boveri  dis- 
covered, they  still  open  in 
the  corresponding  part  of 
the  secondary  "mantle- 
cavity." 

In  other  higher  verte- 
brates, also,  the  kidneys  de- 
velop (though  verydifferently 
formed  later  on)  from  similar 
structures,  which  have  been  secondarily  derived  from  the 
segmental  pronephridia  of  the  acrania.  The  parts  of  the 
mesoderm  at  which  the  first  traces  of  them  are  found  are 
usually  called  the  middle  or  mesenteric  plates,  and  their 
segmental  parts  mesornera.  As  the  first  traces  of  the  gonades 
make  their  appearance  in  the  ccelous  epithelium  o(  these 
middle  plates  nearer  inward  (or  the  middle)  from  the  inner 
funnels  of  the  nephro-canals,  it  is  better  to  count  this  part  of 
the  mesoderm  with  the  hyposoma. 


Fig.  176.  Transverse  section  of 
the  trunk  of  a  primitive  vertebrate 
(prospondylusj.  a  aorta,  6  lateral  furrow 
(prorenal  duct),  rf small  intestine,/" float- 
ing border  of  the  skin,  i  muscular  cavity 
(dorsal  ccelom-pouch),  ms  muscles,  n 
renal  canals,  11  outer  skin,  r  spinal 
marrow,  s  sexual  glands  (gonades),  / 
corium,  -'  principal  vein,  v  chorda. 


348 


THE  ARTICULATION  OF  THE  BODY 


The  chief  and  oldest  organ  of  the  vertebrate  hyposoma, 
the  alimentary  canal,  is  generally  described  as  an  unsegmented 
organ.  But  we  could  just  as  well  say  that  it  is  the  oldest  of 
all  the  metamerous  organs  of  the  vertebrate  ;  the  double  row 
of  the  ccelom-pouches  grows  out  of  the  dorsal  wall  of  the  gut, 
on  either  side  of  the  chorda.  In  the  brief  period  during  which 
these  segmental  ccelom-pouches  are  still  openly  connected 
with  the  gut,  they  look  just  like  a  double  chain  of  metamerous 
visceral  glands.  But  apart  from  this,  we  have  originally  in 
all  vertebrates  an  important  articulation  of  the  fore-gut,  that 
is  wanting  in  the  lower  gut,  the  segmentation  of  the  branchial 
gut,  or  "  branchiomerism." 


V     Hid   p 


Fig.  177.— Optical  longitudinal  section  of  the  primitive  vertebrate 

( prospondyljts  ).  a  aorta,  of  anus,  au  eye,  d  small  intestine,  e  parietal  eye 
(epiphysis),/' floating  border  of  skin,  g  auditor}'  vesicle, gh  brain,  h  heart,  k  gill- 
gut,  ka  branchial  (gill)  artery,  kg  branchial  vascular  arches,  ks  gill-clefts, 
/  liver,  ma  stomach,  nid  mouth,  ms  muscles,  na  nose  (olfactory  pit),  o  outer  skin, 
p  gullet,  r  spinal  cord,  s  sexual  glands  (gonades),  t  corium,  v  principal  vein, 
x  chorda,  y  hypophysis  (urinary  appendage). 

The  gill-clefts,  which  originally  in  the  older  acrania  pierced 
the  wall  of  the  fore-gut  and  the  gill-arches  that  separated 
them,  were  presumably  also  segmental,  and  distributed  among 
the  various  metamera  of  the  chain,  like  the  gonades  in  the 
after-gut  and  the  nephridia  (Fig.  177  ksj.  In  the  amphioxus, 
too,  they  are  still  segmentally  formed.  Probably  there  was  a 
division  of  labour  of  the  hyposomites  in  the  older  (and  long 
extinct)  acrania,  in  such  wise  that  those  of  the  fore-gut  took 
the  function  of  breathing  and  those  of  the  after-gut  repro- 
duction. The  former  developed  into  gill-pouches,  the  latter 
into  sex-pouches.  There  may  have  been  pronephridia  in 
both.  Branchiomerism  is  so  much  changed  in  the  living 
vertebrates,  and  so  reduced  in  the  amniotes,  that  it  has  been 


THE  ARTICULATION  OF  THE  BODY 


denied  altogether  by  some  scientists.  Moreover,  in  the 
amniotes  their  respiratory  function  has  disappeared.  Never- 
theless, certain  parts  of  them  have  been  generally  maintained 
in  the  embryo  by  a  tenacious  heredity. 

At  a  very  early  stage  we  notice  in  the  embryo  of  man  and 
the  other  amniotes,  at  each  side  of  the  head,  the  remarkable 
and  important  structures  which  we  call  the  gill-arches  and 
-ill-clefts  (Plates  VIII. -XIII.,  Figs.  178-1S1/O.  Theybelong 
to  the  characteristic  and  inalienable  organs  of  the  amniote- 
embrvo,  and  are  found  always  in  the  same  spot  and  with  the 
same  arrangement  and  structure.  There  are  formed  to  the  right 
and  left  in  the  lateral  wall  of  the  fore-gut  cavity,  in  its  fore- 
most part,  first  a  pair  and  then  several  pairs  of  sac-shaped 
inlets,  that  pierce  the  whole  thickness  of  the  lateral  wall  of 
the  head.  They  are  thus  converted  into  clefts,  through  which 
one  can  penetrate  freely  from  without  into  the  gullet.  The 
wall  thickens  between  these  branchial  folds,  and  changes  into 
an  arch-like  or  sickle-shaped  piece — the  gill,  or  gullet-arch. 
In  this  the  muscles  and  skeletal  parts  of  the  branchial  gut 
separate  ;  a  blood-vessel  arch  arises  afterwards  on  their  inner 
side  (Fig.  177  kaj.  The  number  of  the  branchial  arches  and 
the  clefts  that  alternate  with  them  is  four  or  five  on  each  side 
in  the  higher  vertebrates  (Fig.  181  d, /,/',/  )•  In  some  of 
the  fishes  (selachii)  and  in  the  cyclostoma  we  find  six  or  seven 
of  them  permanently. 

These  remarkable  structures  had  originally  the  function 
of  respiratory  organs — gills.  In  the  fishes  the  water  that 
serves  for  breathing  and  is  taken  in  at  the  mouth  still  always 
passes  out  by  the  branchial  clefts  at  the  sides  of  the  gullet. 
In  the  higher  vertebrates  they  afterwards  disappear.  The 
branchial  arches  are  converted  partly  into  the  jaws,  partly 
into  the  bones  of  the  tongue  and  the  car.  From  the  first 
gill-cleft  is  formed  the  tympanic  cavity  of  the  ear.  (Cf.  Plates 
I.,  VIII.   XIII.,  first  and  second  row.) 

The  primary  articulation  of  the  vertebrate  body,  which 
proceeds  from  the  primitive  segments  of  the  mesoderm,  affects 
most  of  its  chief  systems  of  organs  :  in  the  episoma  especially 
the  muscles  and  skeleton,  in  the  hyposoma  the  kidneys  and 


THE  ARTICVLATIOX  OF  THE  BODY 


gonades  and  the  branchial   gut.     Then  there  is  a  secondary 
articulation  of  other  systems  of  organs,  which  is  dependent 


Fig.    178.—  Head  of  a  shark  embryo   ffiristiurusj,  eight  mm.  long, 

magnified  Lwcnty  tinier.     (From  Parker.)     Seen  From  the  ventral  side. 


Fig.  179. 
Figs.    179  and  1S0.— Head   of   a   chick 

embryo,   of   the    third    day.       Fig,    179    from 
the  front,    Fig.    1S0    from    the    right,      n    rudi- 
mentary   nose     (olfactory    pit),   /   rudimentary  pIG    ,g,_ 
eye    (optic     pit,    lens-cavity),  g    rudimentary 

ear  (auditory  pit),  v  fore-brain,  gl  eye-cleft  Of  the  three  pairs  of  gill-arehes 
the  first  has  passed  into  a  process  of  the  upper  jaw  (o)  and  of  the  lower  jaw 
(u).     (From  Kolliker.) 

FlG.  181.— Head  Of  a  dog  embryo,  seen  from  the  front.  it  the  two  lateral 
halves  of  the  foremost  cerebral  vesicle,  b  rudimentary  eye,  c  middle  cerebral 
vesicle,  de  first  pair  of  gill-arches  (e  upper-jaw  process,  d  lower-jaw  process), 

/■/'■/''  second,  third,  and  fourth  pairs  of  gill-arches,  g  //  //-heart  [gright, 
h  left  auricle  ;  i  left,  k  right  ventricle  I,  /  origin  of  the  aorta  with  three  pairs  of 
arches,  which  go  to  the  gill-arches.      (From  Bischoff. ) 


THE  ARTICULATION  OF  THE  BODY 


on  and  determined  by  the  preceding  one.  Thus  we  have  in 
the  later  stages  the  development  of  a  segmental  arrangement 

o\  the  peripheral  nerves  and  blood-vessels  ;  the  one  starts 
from  the  episoma,  the  other  from  the  hvposoma.  Especially 
important  is  the  fact  that  in  man  and  all  other  vertebrates  the 
psychic  organ  is  subject  to  this  "secondary  metamerism." 
It  is  readily  recognisable  in  the  human  embryo  in  the  fourth 
week,  the  eetodermic  nerve-roots  connecting  with  the  corres- 
ponding mesodermic  muscle-plates  of  the  provertebrae  (Fig. 
iSj). 

There  are  few  parts  of  the  vertebrate  organism  that  are 
not  subject  to  metamerism,  like  the  outer  covering-  or 
integument  oi  the  body.  The  outer  skin  (epidermis )  is 
unsegmented  from  the  first,  and  proceeds  from  the  uniformly 
disposed  horny  plate.  Moreover,  the  underlying  cutis  is  also 
not  metamerous,  although  it  developes  from  the  segmental 
structure  of  the  cutis-plates  (or  lateral  lamina?  of  the  episo- 
mites,  Figs.  1 68,  169  cp).  The  vertebrates  are  strikingly  and 
profoundly  different  from  the  articulates  in  these  respects  also. 
Further,  most  of  the  vertebrates  still  have  a  number  of 
unarticulated  or  monomeric  organs,  which  have  arisen  loeallv, 
by  adaptation  of  particular  parts  of  the  body  to  certain  special 
functions.  Of  this  character  are  the  sense-organs  in  the 
episoma,  and  the  limbs,  the  heart,  the  spleen,  and  the  large 
visceral  glands — lungs,  liver,  pancreas,  etc. — in  the  hvposoma. 
The  heart  is  originally  only  a  local  spindle-shaped  enlarge- 
ment of  the  large  ventral  blood-vessel  or  principal  vein,  at 
the  point  where  the  subintestinal  passes  into  the  branchial 
artery,  at  the  limit  oi  the  head  and  trunk  (figs.  iSt,  [82). 
The  three  higher  sense-organs — nose,  eye,  and  ear — were 
originally  developed  in  the  same  form  in  all  the  craniotes,  as 
three  pairs  of  small  depressions  in  the  skin  at  the  side  of  the 
head. 

The  organ  of  smell,  the  nose,  has  the  appearance  of  a 
pair  of  small  pits  above  the  mouth-aperture,  in  front  of  the 
head  (Fig.  180  //)•  The  organ  oi  sight,  the  eye,  is  found  at 
the  side  of  the  head,  also  in  the  shape  of  a  depression  (Figs. 
180  /,  1S1  b),  to   which  corresponds  a  considerable  vesicular 


THE  ARTICULATION  OF  THE  BODY 


hollowing  of  the  foremost  cerebral  vesicle  on  each  side. 
Farther  behind,  at  each  side  of  the  head,  there  is  a  third 
depression,  the  first  trace  of  the  organ  of  hearing  (Fig.  iSog-). 


Rudiment  of  ear 
(labyrinthic  vesicles) 


Pneumogastric  nerve 
X.    Vagus 


Terminal  nerve 
XI.  Accessorius 


Twentieth  spinal  nerve 

Fig.  182.— Human  embryo  of  the  fourth'week  (twenty-six  days  old), 

six  mm.  long-,  magnified  twenty  times.  (From  Moll.)  The  rudiments' of  the 
cerebral  nerves  and  the  roots  of  the  spinal  nerves  are  especially  marked. 
Underneath  the  four  gill-arches  (left  side)  is  the  heart  (with  auricle,  I",  and 
ventricle,  A"),  under  this  again  the  liver  ( L). 

As  yet  we  can  see  nothing  of  the  later  elaborate  structure  of 
these  organs,  nor  of  the  characteristic  build  of  the  face  (cf. 
Plate  I.,  Figs.  1-5). 

When    the   human    embryo    has    reached    this    stage    of 


THE  ARTICULATION  OF  THE  /,'()/)  Y 


development,  it  can  still  .scarcely  be  distinguished  from  that 
o(  any  other  higher  vertebrate  (c\.  Plate  1.  and  p.  356).  All 
the  chief  parts  o(  the  body  are  now  laid  down  :  the  head 
with  the  primitive  skull,  the  rudiments  of  the  three  higher 
Sense-organs  and  the  live  cerebral  vesicles,  and  the  ^ill- 
arches  and  clefts  ;  the  trunk  with  the  spinal  cord,  the  rudiment 
of  the  vertebral  column,  the  chain  of  metamera,  the  heart  and 
chief  blood-vessels,  and 
the  kidneys.  At  this 
Stage  man  is  a  higher 
vertebrate,  but  shows 
no  essential  morpho- 
logical difference  from 
the  embryo  of  the 
mammals,  the  birds, 
the  reptiles- etc.  (cf.  p. 
356,  Plates  VJU.-XIIL, 
top  row).  This  is  an 
ontogenetic  fact  of  the 
utmost  significance. 
From  it  we  can  gather 
the  most  important 
phylogenetic  conclu- 
sions. 

There  is  still  no  trace 
of  the  limbs.  Although 
head  and  trunk  are 
separated  and  all  the 
principal  internal 
organs  are  laid  down, 
there  is  no  indication 
whatever  of  the  "extremities"  at  this  stage;  they  are 
formed  later  on.  Here  again  we  have  a  fact  of  the  utmost 
interest.  It  proves  that  the  older  vertebrates  had  no 
feet,  as  we  find  to-day  in  the  lowest  living  vertebrates 
(amphioxus  and  the  cvclostoma).  The  descendants  of 
these  ancient  footless  vertebrates  only  acquired  extremities 
— two     fore-legs     and     two     hind-legs — at     a     much      later 

2A 


i"ic;.  183.  Transverse  section  of  the 
shoulder  and  fore-limb  (wing)  of  a  chick- 
embryo  of  the  fourth  day,  magnified  about 
twenty  times.  Beside  the  medullary  tube  we 
ran  seeon  each  side  three  clear  streaks  in  the 
dark  dorsal  wall,  which  advance  into  the 
rudimentary  fore-limb  or  wing  1 1 1.  The  upper- 
most of  them  is  the  muscular  plate;  tin-  middle 
is  the  hind  and  the  lowest  On-  lor,'  rool  of  a 
spinal  nerve.  Under  the  chorda  in  the  middle 
is  1  ho  single  aorta,  and  at  each  side  o(  ii  a 
cardinal  vein,  and  below  these  the  primitive 
kidneys.  The  gut  is  almost  closed, 
ventral  wall  advances  into  the  amnion, 
encloses  the  embryo.     (From  Remai.) 


The 
hich 


THE  ARTICULATION  OF  THE  BODY 


stage  of  development.  These  were  at  first  all  alike, 
though  they  afterwards  vary  considerably  in  structure — 
becoming  fins  (of  breast  and  belly)  in  the  fishes,  wings  and 
legs  in  the  birds,  fore  and  hind  legs  in  the  creeping  animals, 
arms  and  legs  in  the  apes  and  man.  All  these  parts  develop 
from  the  same  simple  original  structure,  which  forms 
secondarily  from  the  trunk-wall  (Figs.  183,  184).  They 
have     always     the    appearance     of     two     pairs     of     small 

birds,  which  re- 
present at  first 
simple  roundish 
knobs  or  plates. 
Gradually  each  of 
these  plates  be- 
comes a  large 
projection,  in 
which  we  can  dis- 
tinguish a  small 
inner  part  and  a 
broader  outer 
part.  The  latter 
is  the  rudiment 
of  the  foot  or 
hand,  the  former 
that  of  the  leg  or 
arm.  The  simi- 
larity of  the 
original  rudiment 
of  the  limbs  in 
different  groups 
of  vertebrates  will  be  seen  on  Plates  VIII. -XIII. 

How  the  five  fingers  or  toes  with  their  blood-vessels 
gradually  differentiate  within  the  simple  fin-like  structure  of 
the  limbs  can  be  seen  in  the  instance  of  the  lizard  in  Fig.  185. 
They  are  formed  in  just  the  same  way  in  man  ;  in  the  human 
embryo  of  five  weeks  the  five  fingers  can  clearly  be  distin- 
guished within  the  fin-plate  (Fig.  186). 

The   careful   study  and   comparison    of  human    embryos 


Fig.  184.— Transverse  section  of  the  pelvic 

region  and  hind  legs  of  a  chick-embryo  of  the  fourth 
day,  magnified  about  forty  times,  h  horn-plate, 
•hj  medullary  tube,  n  Canal  of  the  tube,  u  primitive 
kidneys,  x  chorda,  e  hind  legs,  b  allantois  canal  in  the 
ventral  wall,  /  aorta,  v  cardinal  veins,  a  gut,  d  gut- 
gland  layer,  /'gut  fibre  layer,  ^embryonic  epithelium, 
r  dorsal  muscles,  c  body-cavity  or  cceloma.  (From 
Waldeyer. ) 


THE  ARTICULATION  OF  THE  BODY 


with  those  of  other  vertebrates  at  this  stage  o(  development  is 

very  instructive,  and  reveals  more  mysteries  to  the  impartial 
student  than  all  the  religions  in  the  world  put  together.  For 
instance,  let  us  compare  attentively  the  three  successive 
Stages    of    development    that    are    represented,    in    twenty 


Fig.  185.  Development  of  the  lizard's  legs  (lacerta  agUis),  with 
special  relation  10  their  blood-vessels.  r,  j, 5,  7,  9,  //  right  fore-leg;  /,;.  /■; 
left  fore-leg  ;  -'.  -/.  '',  8,  m,  u  right  hind-leg  ;  14,  16  left  hind-leg  ;  SS I '  lateral 
veins  of  the  trunk,  VU  umbilical  vein.     (From  /•'.  ffochstetter.) 

different  amniotes,  in  the  six  following  Plates  (VIII. -XI II.). 
When  we  see  that  as  a  fact  twenty  different  amniotes  of  such 
divergent  characters  develop  from  the  same  embryonic  form, 
we  can  easily  understand  that  they  may  all  descend  from  a 
common  ancestor. 


356 


THE  ARTICULATION  OF  THE  BODY 


In  the  first  stage  of  development  (the  first  row,  I.),  in 
which  the  head  with  the  five  cerebral  vesicles  is  already 
clearly  indicated,  but  there  are  no  limbs,  the  embryos  of  all 
the  vertebrates,  from  the  fish  to  man,  are  only  incidentally  or 
not  at  all  different  from  each  other.     In  the  second  stage  (the 


Fig.  i86. — Human  embryo,  five  weeks  old,  eleven  mm.  long-,  seen  from 
the  right,  magnified  ten  times.  (  From  Russel  Bardeen  and  Harmon  Lewis. )  In 
the  undissected  head  we  see  the  eye,  mouth,  and  ear.  In  the  trunk  the  skin 
and  part  of  the  muscles  have  been  removed,  so  that  the  cartilaginous  vertebral 
column  is  free  ;  the  dorsal  root  of  a  spinal  nerve  goes  out  from  eaeli  vertebra 
(towards  the  skin  of  the  back).  In  the  middle  of  the  lower  half  of  the  figure 
part  of  the  ribs  and  intercostal  muscles  are  visible.  The  skin  and  muscles  have 
also  been  icmoved  from  the  right  limbs;  the  internal  rudiments  of  the  ft\o 
Fingers  of  the  hand,  and  five  toes  of  the  foot,  are  clearly  seen  within  the  fin- 
shaped  plate,  and  also  the  strong  network  of  nerves  that  goes  from  the  spinal 
cord  to  the  extremities.  The  tail  projects  under  the  foot,  and  to  the  right  of 
it  is  the  first  part  of  the  umbilical  cord. 


THE  ARTICULATION  OF  THE  BODY 


middle  row,  II.).  which  shows  the  limbs,  we  begin  to  see 
differences  between  the  embryos  of  the  lower  and  higher 
vertebrates  ;  but  the  human  embryo  is  still  hardly  distin- 
guishable from  that  of  the  higher  mammals.  In  the  third 
Stage     (lowest     row,    III.),    in    which    the    gill-arches    have 

nk  1 


Fig.  187. 

I'u.-.  187  <).  Embryos  of  the  bat  (,ves- 
pertilio  murinus)  at  three  different  stages. 
(From  Oscar  Schultze.)  Fit;.  1S7.  Rudimentary 
limbs  if  fore-leg,  //  hind-leg).  I  lenticular 
depression,  r  olfactory  pit,  nk  upper  jaw,  nk 
lower  jaw,  k...  £„,  k- ,  first,  second,  and  third 
gill-arches,  »  amnion,  >i  umbilical  vessel,  </ 
yelk-sac.  Fig.  iSS.  Rudiment  of  flying  mem- 
brane, membranous  told  between  ion' and  hind 
leg.  "  umbilical  vessel,  0  ear-opening, /"flying 
membrane.  Fig.  189.  The  flying  membrane 
developed  ami  stretched  aero—  the  fingers  of 
the  hands,  which  cover  the  face. 

disappeared  and  the  face  is  formed, 
the  differences  become  more  pro- 
nounced. These  are  facts  of  a 
significance  that  cannot  be  exag- 
gerated.' 

If   there    is   an   intimate  causal    connection    between    the 
processes    of    embryology    and    stem-history,    as   we    must 

1  Because  they  show  how  the  most  diverse  structures  may  be  developed 
from  a  common  form.  As  we  actually  see  this  in  tin'  case  of  the  embryos,  we 
have  a  righl  to  assume  it  of  the  stem-forms.     Nevertheless,  this  resemblance, 

however  great,  is   never  a  real   identity.      Even   the  embryos  of  the   different 
individuals  of  one  species  are  usually  not  really  identical. 


Fig.  189. 


358  THE  ARTICULATION  OF  THE  BODY 


assume  in  virtue  of  the  laws  of  heredity,  several  important 
phylogenetic  conclusions  follow  at  once  from  these  onto- 
genetic facts.  The  profound  and  remarkable  similarity  in 
the  embryonic  development  of  man  and  the  other  vertebrates 
can  only  be  explained  when  we  admit  their  descent  from  a 
common  ancestor.  As  a  fact,  this  common  descent  is  now 
accepted  by  all  competent  scientists  ;  they  have  substituted 
the  natural  evolution  for  the  supernatural  creation  of 
organisms. 


REPTILE   EMBRYOS 


iution  of  Man  J.Ed 


PI  IE 


E. Lizard 

A.  Snake 

K.Crocodile 

Lacerta 

Col1 

Alligator 

SAUROPSIDA     EMBRYOS 


v  ...... 


■r ,/ 


ft' 


,• 


P 


SAUROPSi  DA     EMBRYOS 


•f^ 


,)  R  K  iwi 

Hat: 


MAMMAL    EMBRYOS 


/ 


dna 


Iphin 

Phocaena 


bbon 


MAMMAL   EMBRYOS. 


ran.V.Fd. 


PI  ffl 


B.  Marsupial 

Didel; 


Sus. 


Capreolus. 


R.Oj 


MAMMAL    EMBRYOS. 


vol  lit  ion 


XT 


M  I 


F.  !■ 

iphus 


I.  Hare 
Lepus 


M  N!d! 
Homo 


EXPLANATION   OF    PLATES   VIII. -XIII. 

Six  comparative  plates  of  twenty  amniote-embryos  of  fifteen 
different  orders. 

The  six  plates  VIII. -XIII.  show  the  more  or  less  significant  similarity  that 
exists,  in  respect  of  most  important  structural  features,  between  the  human 
embryo  and  the  embryo  of  the  higher  vertebrates  (amniotes)  in  the  earlier 
periods  of  development.  The  similarity  is  closer  the  earlier  the  stage  of 
development  at  which  we  compare  them.  It  persists  the  longer  the  closer  is 
the  stem-relation  between  the  various  animals,  in  harmony  with  the  "law  of 
the  ontogenetic  connection  of  related  forms"  (see  following  Chapter). 

Plates  VIII.,  IX.,  and  X.  represent  the  embryos  of  nine  different  sauropsids 
-  six  reptiles  and  three  birds  —at  throe  different  stages. 

l'latos  XI.,  XII.,  and  XIII.  show  the  embryos  of  eleven  different  mammals 
of  the  three  corresponding  stages.  The  conditions  of  tin'  three  different 
stages!  represented  by  the  three  rows  (I.,  II.,  III.),  are  chosen  so  as  to 
correspond  as  closely  as  possible. 

The  first  nop)  row,  I.,  represents  an  early  stage,  with  gill-clefts,  without 
leys.  The  second  (middle)  row,  II.,  shows  a  somewhat  later  stage,  with  the 
first  traces  o\'  the  leys,  still  with  gill-clefts.  The  third  (bottom)  row  represents 
a  -.till  later  stage,  with  more  advanced  leys,  after  the  disappearance  of  the 
gill-clefts.  The  envelopes  and  appendages  ol  the  embryo  (amnion,  yelk-sac, 
allantois)  are  omitted.  The  whole  of  the  sixty  figures  are  slightly  magnified, 
the  lower  ones  less  than  the  upper.  They  have  been  almost  reduced  to  a 
common  size  for  the  purpose  of  comparison.  All  the  embryos  are  looked  al 
from  the  left  :  the  head-end  is  upward,  the  tail-end  below,  the  curved  back 
turned  to  the  ritfht.  The  letters  have  the  same  meaning  in  all  the  sixty  figures  : 
:•  Fore-brain,  j  intermediate-brain,  in  middle-brain,  /;  hind-brain,  n  after- 
brain,  r  spinal  cord,  e  nose,  a  eye,  "  ear,  k  gill-arches,  ,-  heart,  m  vertebral 
column,  /'fore  ley,  b  hind  ley,  s  tail. 


Stem-reptile  (hatteria)  D. 
Lizard  (lactrta)  E. 
Serpent  (coluber J  A. 
Crocodile  (alligator )  A". 
Tortoise  (chelone)  T. 
River-tortoise  ( Irionyx  )  J. 
Wen  (  gall  its)  G. 
Kiwi  (apteryx)  V. 
Ostrich  (struthio)  Z. 
Sea  urchin  (echidna )  V. 


Opossum  (didelphys)  B. 
Dolphin  (phoctena)  P. 

Pig  ( sns)  S. 

Goat  (capreolus)  C. 

Ox  (bos)  R. 
Hoy  (ranis )  11. 

Hat  (rhinolophus)  /•'. 
Hare  (lepus)  /.. 
Gibbon  (hylobates)  X. 
.Man  ( homo )  .1/. 


FOURTEENTH  TABLE 

SYNOPSIS  OF  THE  FUNDAMENTAL  ANTITHESIS 

IN  THE  ORGANISATION  AND  ARTICULATION 

OF  THE  VERTEBRATES  AND  ARTICULATES 


Vertebration  of  the  Vertebrates 

(Acrania  and  Craniota). 


Articulation  of  the  Articulates 

(Annelida,  Crustacea,  Tracheata). 


t.  Epidermis    without    Cuticula, 

not     articulated,    without     chitine- 
covering. 

2.  Skeleton  axial,  with  Chorda 

and  chorda-sheath. 

(Internal  axial  skeleton.) 

3.  Musculature    periskeletal 

(formed   of  the  wall  of  the  hollow 
ccelom-pouches,  with  myocoel). 

4.  Nervous  centre  dorsal,  origi- 
nally unarticulated  (spinal  marrow). 

(Simple  medullary  tube.) 

5.  Heart  ventral,  arising  from  the 
ventral  vessel  of  the  vermalia. 

6.  Gut  with  gill-chamber  (head- 
gut  converted  into  a  gill-pannierj 
with  gill-clefts   and   hypobranchial 

groove). 

7-  Nephridia,  originally  segmental, 
with  myoccel-connection,  and  with 
primary  prorenal  duct. 

8.  Gonades,  originally  segmental, 
formed  from  the  visceral  meso- 
blast. 

9-  Body-cavities  (right  and  left) 
early  divided  by  a  frontal  Sep- 
tum into  a  dorsal  myocoel  and  a 
ventral    gonocoel  (episomites   and 

hyposomites). 


Epidermis  with  euticular  mail 

(composed  of  chitine,  articulated). 


2.  Skeleton   tegmental,   without 

Chorda  and  without  chorda  sheath. 
(External  euticular  skeleton. ) 

3.  Musculature  endoskeletal 

(formed  of  solid  mesodermic 
streaks,  without  myocoel). 

4.  Nervous  centre  ventral,  origi- 
nally articulated  (ventral  marrow). 
(Doublechain  of  ventral  ganglia. ) 

5.  Heart  dorsa  developing  from 
the  dorsal  vessel  of  the  vermalia. 

6.  Gut  without  gill-chamber  1  head- 
gut  never  with  gill-clefts  ;  hypo- 
branchial  groove  wanting  in  all 
the  articulates). 

7.  Nephridia,  originally  segmental, 
without  myoccel-connection,  and 
without  primary  prorenal  duct. 

S.   Gonades,     originally     segmental, 

formed  from  the  parietal  meso- 
blast. 

9.    Body-Cavities     (right     and     left) 

without  frontal  septum  :  hence 

no  division  into  dorsal  episomites 
and  ventral  hyposomites  ;  no  anti- 
thesis of  dorsal  and  ventral  body. 


360 


CHAPTER   XV. 

FCETAL  MEMBRANES  AND  CIRCULATION1 

Tin-  mammal-organisation  of  man.  Man  has  the  same  structure  a?,  all  the 
other  mammals,  and  his  embryo  developes  in  the  same  way  as  that  of  the 
higher  vertebrates.  The  law  of  the  ontogenetic  connection  of  related 
forms.  Application  of  it  to  man.  Shape  and  sizo  of  the  human 
embryo  in  the  firsl  lour  weeks.  The  human  embryo  is  almost 
completely  like  thai  of  other  mammals  in  structure  in  the  first  month.  In 
tin-  second  month  certain  notable  differences  begin  to  appear.  Tin- 
appendages  and  envelopes  of  the  human  embryo.  Yelk-sac  or  umbilical 
vesicle.  AJUantois  or  urinary  sac.  Placenta.  Ventral  pedicle  and  peculiar 
placentation  of  man  and  the  anthropoid  apes.  Amnion  and  serok-mma 
(serous  membranes).  Exoccelom.  The  heart,  the  first  blood-vessels,  and 
the  blood  are  formed  from  the  gut-fibre  layer.  Vascular  layer  and 
mesencyhma.  The  heart  separates  from  the  wall  of  the  fore-gut  Double 
structure  of  the  ln-art  in  the  amniotes  cenogenetic.  Tin-  first  embryonic 
circulation  in  the  germinative  area  :  vitelline  arteries  and  veins.  The  second 
embryonic  circulation  in  the  allantois :  umbilical  arteries  and  veins.  Sections 
of  human  embryology. 

Among  the  many  interesting  phenomena  that  we  have 
encountered  in  the  course  of  human  embryology,  there  is  an 
especial  importance  in  the  fact  that  the  development  of  the 
human  body  follows  from  the  beginning  just  the  same  lines 
as  that  of  the  other  viviparous  mammals.  As  a  fact,  all  the 
embryonic  peculiarities  that  distinguish  the  mammals  from 
other  animals  are  found  also  in  man  ;  even  the  ovum  with  its 
distinctive  membrane  (zona  pelhtcida,  Fig.  14)  shows  the 
same  typical  structure  in  all  mammals  (apart  from  the  older 
oviparous  monotremes).  It  has  long  since  been  deduced 
from  the  structure  of  the  developed  man  that  his  natural 
place  in  the  animal  kingdom  is  among  the  mammals.  Linne 
(1735)  placed  him  in  this  class  with  the  apes,  in  one  and  the 
same  order  (primates ),  in  his  Systema  natures.  This 
position  is  fully  confirmed  by  comparative  embryology.     We 

'  Cf.  Sir  \V.  Turner:  "Some  general  observations  on  tin-  placenta,  with 
especial  reference  to  the  theory  of  evolution, "Journal  of  Aunt,  and  Physiol. 
11S771:  and  "  On  the  placentation  of  the  apes,  with  a  comparison  with  that   of 
tin-  human  female,"  /'////".v.  Trans.,  1878,  vcL  169. 
361 


362  FCETAL  MEMBRANES  AXD  CIRCULATION 

see  that  man  entirely  resembles  the  higher  mammals,  and 
most  of  all  the  apes,  in  embryonic  development  as  well  as  in 
anatomic  structure.  And  if  we  seek  to  understand  this  onto- 
genetic agreement,  in  the  light  of  the  biogenetic  law,  we  find 
that  it  proves  clearly  and  necessarily  the  descent  of  man  from 
a  series  of  other  mammals,  and  proximately  from  the 
primates.  The  common  origin  of  man  and  the  other 
mammals  from  a  single  ancient  stem-form  can  no  longer  be 
questioned  ;  nor  can  the  immediate  blood-relationship  of  man 
and  the  ape. 

The  essential  agreement  in  the  whole  bodily  form  and 
inner  structure  is  still  visible  in  the  embryo  of  man  and  the 
other  mammals  at  the  late  stage  of  development  at  which 
the  mammal-body  can  be  recognised  as  such.  (Cf. 
Plates  VIII. -XIII.,  second  row.)  But  at  a  somewhat  earlier 
stage,  in  which  the  limbs,  gill-arches,  sense-organs,  etc.,  are 
already  outlined,  we  cannot  yet  recognise  the  mammal 
embryos  as  such,  or  distinguish  them  from  those  of  birds  and 
reptiles  (Plates  VIII. -XIII.,  top  row).  When  we  consider 
still  earlier  stages  of  development,  we  are  unable  to  discover 
any  essential  difference  in  bodily  structure  between  the 
embryos  of  these  higher  vertebrates  and  those  of  the  lower, 
the  amphibia  and  fishes.  If,  in  fine,  we  go  back  to  the 
construction  of  the  body  out  of  the  four  germinal  layers,  we 
are  astonished  to  perceive  that  these  four  layers  are  the 
same  in  all  vertebrates,  and  everywhere  take  a  similar 
part  in  the  building-up  of  the  fundamental  organs  of  the 
body.  If  we  inquire  as  to  the  origin  of  these  four  secondary 
layers,  we  learn  that  they  always  arise  in  the  same  way  from 
the  two  primary  layers ;  and  the  latter  have  the  same 
significance  in  all  the  metazoa  (i.e.,  all  animals  except  the 
unicellulars).  Finally,  we  see  that  the  cells  which  make  up 
the  primary  germinal  layers  owe  their  origin  in  every  case  to 
the  repeated  cleavage  of  a  single  simple  cell,  the  stem-cell 
or  fecundated  ovum. 

It  is  impossible  to  lay  too  much  stress  on  this  remarkable 
agreement  in  the  chief  embryonic  features  in  man  and  the 
other  animals.     We  shall   make  use  of  it  later  on  for  our 


FCET.  1 1.  MEMBR.  I NES  .  I ND  ( 7A'<  7  /..  I  TION 

monophyletic  theory  of  descent — the  hypothesis  of  a  common 
descent  o(  man  and  all  the  metazoa  from  the  gastraea.     The 

first  rudiments  of  the  principal  parts  o\  the  body,  especially 
the  oldest  organ,  the  alimentary  canal,  are  the  same  every- 
where ;  they  have  always  the  same  extremely  simple  form. 
All  the  peculiarities  that  distinguish  the  various  groups  of 
animals  from  each  other  only  appear  gradually  in  the  course 
of  embryonic  development  ;  and  the  closer  the  relation  of  the 
various  groups,  the  later  they  are  found.  We  may  formulate 
this  phenomenon  in  a  definite  law,  which  may  in  a  sense  be 
regarded  as  an  appendix  to  our  biogenetic  law.  This  is  the 
law  of  the  ontogenetic  connection  of  related  animal  forms.  It 
runs:  The  closer  the  relation  of  two  fully-developed  animals 
in  respect  of  their  whole  bodily  structure,  and  the  nearer  they 
are  connected  in  the  classification  of  the  animal  kingdom,  the 
longer  does  their  embryonic  form  retain  its  identity,  and  the 
longer  it  is  impossible  (or  only  possible  on  the  ground  of 
subordinate  features)  to  distinguish  between  their  embryos. 
This  law  applies  to  all  animals  whose  embryonic  develop- 
ment is,  in  the  main,  an  hereditary  summary  of  their 
ancestral  history,  or  in  which  the  original  form  of  develop- 
ment has  been  faithfully  preserved  by  heredity.  When,  on 
the  other  hand,  it  has  been  altered  by  cenogenesis,  or  disturb- 
ance of  development,  we  find  a  limitation  of  the  law,  which 
increases  in  proportion  to  the  introduction  of  new  features  by 
adaptation  (cf.  Chapter  I.,  pp.  8-10).  Thus  the  apparent 
exceptions  to  the  law  can  always  be  traced  to  cenogenesis. 

When  we  apply  to  man  this  law  of  the  ontogenetic  con- 
nection of  related  forms,  and  run  rapidly  over  the  earliest 
stages  of  human  development  with  an  eye  to  it,  we  notice 
first  of  all  the  morphological  identity  of  the  ovum  in  man  and 
the  other  mammals  at  the  very  beginning  (Figs.  1,  14).  The 
human  ovum  possesses  all  the  distinctive  features  of  the  ovum 
of  the  viviparous  mammals,  especially  the  characteristic 
formation  of  its  membrane  (zona  pellucida),  which  clearly 
distinguishes  it  from  the  ovum  of  all  other  animals.  When 
the  human  foetus  has  attained  the  age  of  fourteen  days,  it 
formsa  globular  vesicle  (or  "  embryonic  vesicle  ")  of  about  four 


364 


FCETAL  MEMBRAXES  AXD  CIRCVLATIOX 


Yelk-sac 


Amnion 


Medullary 

groove 


millimetres  in  diameter.  A  thicker  part  of  its  border  forms 
a  simple  sole-shaped  embryonic  shield  two  millimetres  long 
(Fig.  199).  On  its  dorsal  side  we  find  in  the  middle  line 
the  straight  medullary  furrow,  bordered  by  the  two  parallel 
dorsal  or  medullary  swellings  (in J.  Behind,  it  passes  by  the 
neurenteric  canal  into  the  primitive  gut  or  primitive  groove. 
From  this  the  invagination  of  the  two  ccelom-pouches 
proceeds    in     the    same    way    as    in     the    other    mammals 

(cf.  Figs.  99, 
100).  In  the 
middle  of  the 
sole-shaped  em- 
bryonic shield 
the  first  primi- 
tive segments 
immediately  be- 
gin to  make 
their  appear- 
ance. At  this 
age  the  human 
embryo  cannot 
bedistinguished 
from  that  of 
other  mam- 
mals, such  as 
the  hare  or  dog. 
A  week  later 
(or  after  the 
twenty-first  day) 
the  human  embryo  has  doubled  its  length  ;  it  is  now  about  five 
millimetres  long,  and,  when  seen  from  the  side,  shows  the 
characteristic  bend  of  the  back,  the  swelling  of  the  head-end, 
the  first  outline  of  the  three  higher  sense-organs,  and  the 
rudiments  of  the  gill-clefts,  which  pierce  the  sides  of  the  neck 
(Fig.  191,  III.;  Plate  XIII.,  Fig.  MI).  The  allantois  has 
grown  out  of  the  gut  behind.  The  embryo  is  already  entirely 
enclosed  in  the  amnion,  and  is  only  connected  in  the  middle 
of  the  belly  by  the  vitelline  duct  with  the  embryonic  vesicle, 


Chorion 


Fig.    190.— Sandal-shaped  human   embryo   (or 

sole-shaped  embryonic  shield),  two  mm.  long-,  of  the 
second  week  of  development.  (Cf.  Plates  [V.  and  V.) 
(From  Count  Spec) 


^ 


/■(/■: T.  I /.  MEMBR.  I .\'A.s  .  I ND  C  /A'l  7  /..  I  T/t W  365 


which  changes  into  the  yelk-sac.     There  arc  noextremities  or 

limbs  at  this  stage,  no  trace  o(  arms  or  legs.  The  head-end 
has  been  strongly  differentiated  from  the  tail-end  ;  and  the 
first  outlines  of  the  cerebral  vesicles  in  front,  and  the  heart 
below,  under  the  fore-arm,  are  already  more  or  less  clearly 
seen.  There  is  as  vet  no  real  face.  Moreover,  we  seek  in 
vain  at  this  stage  a  special  character  that  may  distinguish  the 


Fig.  i. ii.  Human  embryos  from  t'.ie  second  to  the  fifteenth  week, 
natural  size,  seen  from  the  left,  the  curved  back  turned  towards  the  right. 
(Mostlj  from  Ecker.)  II.  of  fourteen  days.  III.  of  three  weeks.  IV.  of  Four 
weeks.  V.  of  five  weeks.  VI.  of  six  weeks.  VII.  of  seven  weeks.  VIII.  01 
eight  weeks.     XII.  of  twelve  weeks.     XV.  of  fifteen  weeks. 

human  embryo  from  that  of  other  mammals  (cf.  the  figures  in 
the  top  row  on  Plates  VIII. -XI 1 1.). 

A  week  later  (after  the  fourth  week,  on  the  twenty-eighth 
to  thirtieth  day  of  development)  the  human  embryo  has 
reached  a  length  of  four  to  five  lines,  or  about  a  centimetre 
(Fig.  i()i,  IV.  ;  Plate  XIII.,  Fig.  Mil).  We  can  now 
clearly  distinguish  the  head  with   its  various  parts  ;  inside  it 


366 


FCETAL  MEMBRANES  AXD  CIRCULATION 


the  five  primitive  cerebral  vesicles  (fore-brain,  middle-brain, 
jntermediate-brain,   hind-brain,  and   after-brain)  ;    under   the 


Fig.  i92.^Very  young  human  embryo  of  the  fourth  week,  six  mm. 

long  (taken  from  the  womb  of  a  suicide  eight  hours  after  death).  (From  Rabl.) 
a  nasal  pits,  ,:  eve,  u  lower  jaw,  z  arch  of  bone  of  tongue,  k..  and  kt  third  and 
fourth  gill-arch,  h  heart,  s  primitive  segments,  vg  fore-limb  (arm),  hg hind-limb 
(leg),  between  the  two  the  ventral  pedicle. 


Fig.  193.— Human  embryo  of  the  middle  of  the  fifth  week,  nine  mm. 

long.     (From  Rabl. )      Letters  as  in  Fig.  192,  except  si  bend  of  skull,  ok  upper 
iaw,  lib  neck-indentation. 


FXETAL  MEMBRANES  AND  CIRCULATION 


head  the  gill-arches,  which  divide  the  gill-clefts;  at  the  sides 
o(  the  head  the  rudiments  of  the  eyes,  a  couple  of  pits  in  the 
outer  skin,  with  a  pair  o(  corresponding  simple  vesicles 
growing  out  of  the  lateral  wall  of  the  fore-brain  (Figs.  192, 
[93  a).  Far  behind  the  eyes,  over  the  last  gill-arches,  we  see 
the  vesicular  rudiment  of  the  auscultory  organ.  The  rudi- 
mentary limbs  are  now  clearly  outlined — four  simple  buds  of 
the  shape  oi  round  plates,  a  pair  of  fore  ( Vg)  and  a  pair  of 
hind  legs  f/lffj,  the  former  a  little  larger  than  the  latter.  The 
large  head  bends  over  the  trunk,  almost  at  a  right  angle. 
The  latter  is  still  connected  in  the  middle  of  its  ventral  side 
with  the  embryonic  vesicle  ;  but  the  embryo  has  still  further 
severed  itself  from  it,  so  that  it  already  hangs  out  as  the 
yelk-sac.  The  hind  part  of  the  body  is  also  very  much  curved, 
so  that  the  pointed  tail-end  is  directed  towards  the  head.  The 
head  and  face-part  are  sunk  entirely  on  the  still  open  breast. 
The  bend  soon  increases  so  much  that  the  tail  almost  touches 
the  forehead  (Fig.  191,  V.  ;  Fig.  193).  We  may  then 
distinguish  three  or  four  special  curves  on  the  round  dorsal 
surface — namely,  a  skull-curve  in  the  region  of  the  second 
cerebral  vesicle,  a  neck-curve  at  the  beginning  of  the  spinal 
cord,  and  a  tail-curve  at  the  fore-end.  This  pronounced 
curve  is  only  shared  by  man  and  the  higher  classes  of  verte- 
brates (the  amniotes)  ;  it  is  much  slighter,  or  uot  found  at  all, 
in  the  lower  vertebrates.  At  this  age  (four  weeks)  man  has 
a  considerable  tail,  twice  as  long  as  his  legs.  A  vertical 
longitudinal  section  through  the  middle  plane  of  this  tail 
(Fig.  194)  shows  that  the  hinder  end  of  the  spinal  marrow 
extends  to  the  point  of  the  tail,  as  also  does  the  underlying 
chorda  ( cli J,  the  terminal  continuation  of  the  vertebral 
column.  Of  the  latter,  the  rudiments  of  the  seven  coccygeal 
vertebra'  are  visible — thirty-two  indicates  the  third  and  thirty- 
six  the  seventh  of  these.  Under  the  vertebral  column  we  see 
the  hindmost  ends  of  the  two  large  blood-vessels  of  the  tail, 
the  principal  artery  (aorta  caudalis  or  arteria  sacra/is  media, 
AoJ,and  the  principal  vein  (vena  caudalis  or  sacral  is  media  ). 
Underneath  is  the  opening  of  the  anus  (an)  and  the 
urogenital    sinus  (S.ug).       From   this   anatomic  structure  of 


368 


FCETAL  MEMBRAXES  AXD  CIRCULATION 


the  human  tail  it  is  perfectly  clear  that  it  is  the  rudiment  of 
an  ape-tail,  the  last  hereditary  relic  of  a  long  hairy  tail,  which 
has  been  handed  down  from  our  tertiary  primate  ancestors  to 
the  present  day. 

It  sometimes  happens  that  we  find  even  external  relics  of 
this   tail   growing.     According    to   the    illustrated   works   of 


Fig.    194.— Median  longitudinal  section  of  the  tail  of  a  human 

embryo    fourteen    mm.    long1.     (From    Ross    Granville    Harrison.)      Med 

medullary  tube,  Ca.fil.  caudal  thread,  ch  chorda,  an  caudal  artery,  V.c.i.  caudal 
vein,  mi  anus,  S.  ug  semis  urogenitalis. 

Surgeon-General  Bernhard  Ornstein,  of  Greece,  these  tailed 
men  are  not  uncommon  ;  it  is  not  impossible  that  they  gave 
rise  to  the  ancient  fables  of  the  satyrs.  A  great  number  of 
such  cases  are  given  by  Max  Bartels  in  his  essay  on  "  Tailed 
Men"  (1884,  in  the  Archiv  jiir  Anthropologic,  Band  XV.), 
and  critically  examined.  These  atavistic  human  tails  are 
often  mobile  ;  sometimes  they  contain  only  muscles  and  fat, 


FCETAL  MEMBRANES  AND  CIRCULATION 


369 


sometimes  also  rudiments  o(  caudal  vertebrae.  They  aitain 
a  length  of  jo  25  cm.  and  more.  Granville  Harrison  has  very 
carefully  studied  one  oi  these  eases  o\  "pig-tail,"  which  he 
removed  by  operation  from  a  six  months'  old  child  in  1901. 
The  tail  moved  briskly  when  the  child  cried  or  was  excited, 
and  was  drawn  up  when  at  rest  (Fig".    u>5  A-C). 

In  the  opinion  o(  many  travellers  and  anthropologists,  the 
atavistic  tail-formation  is  hereditary  in  many  isolated  tribes 
.1  c 


Kii..  195.     Tail  of  a  six  months'  old  boy.    -I  stretched  out,  # contracted, 
Cdrawjiout.     it  rom  Granville  Harrison.) 

(especially  in  south-eastern  Asia  and  the  archipelago),  so 
that  we  might  speak  of  a  special  race  or  "species"  of  tailed 
men  (homo  Cauda tus).  Battels  has  "  no  doubt  that  these 
tailed  men  will  be  discovered  in  the  advance  of  our  geo- 
graphical and  ethnographical  knowledge  o(  the  lands  in 
question  "  {A rchvof&r  A nthropologiey  Band  XV.,  p.  129). 

When  we  open  a  human  embryo  of  one  month  (Fig.   196), 
we  find  the  alimentary  canal  formed   in   the  body-cavity,  and 


FCETAL  MEMBRAXES  AXD  CIRCULATIOX 


for  the  most  part  cut  off  from  the  embryonic  vesicle.  There 
are  both  mouth  and  anus  apertures.  But  the  mouth-cavity 
is  not  yet  separated  from  the  nasal  cavity,  and  the  face  not 
yet  shaped.  The  heart  shows  all  its  four  sections  ;  it  is  very 
large,  and  almost  fills  the  whole  of  the  pectoral  cavity 
(Fig.    196   ov).      Behind    it    are    the  very   small    rudimentary 

Fig.  196.— Human  em- 
bryo, four  weeks  old, 

opened  on  the  ventral  side. 
Ventral  and  dorsal  walls 
are  cut  away,  so  as  to  show 
the  contents  of  the  pectoral 
and  abdominal  cavities. 
All  the  appendages  are 
also  removed  (amnion, 
allantois,  yelk-sac),  and 
the  middle  part  of  the  g-ut. 
/;  eye,  3  nose,  4  upper  jaw, 
•7  lower  jaw,  6  second,  6" 
third  gill-arch,  ot'  heart 
(0  right,  o  left  auricle  ;  v 
rig-ht,  v'  left  ventricle),  b 
origin  of  the  aorta,  /'  liver 
in  umbilical  vein),  e  gut 
(with  vitelline  artery,  cut 
off  at  a'),  y  vitelline  vein, 
///  primitive  kidneys,  t 
rudimentary  sexual  glands, 
r  terminal  gut  (cut  off  at 
the  mesentery  e),  11  um- 
bilical artery,  it  umbilical 
\  ein,  Q  fore-leg,  1/  hind-leg. 
(From  Coste. ) 

Fig.  197—  Human  em- 
bryo  five    weeks   old, 

opened  from  the  ventral 
side(as  inFig.  196).  Breast 
and  belly-wall  and  liver  are 
removed.  3  outer  nasal 
process,  4  upper  jaw,  5 
lower  jaw,  e  tongue,  •:• 
right,  v'  left  ventricle  of 
heart,  »'  left  auricle,  b 
origin  of  aorta,  6',  b" ,  b'" 
;'u"  '96-  rK;-  '97-  first,    second,     and    third 

aorta-arches,  c,  <"',  c"  vena  cava,  ae  lungs  (y  pulmonary  artery ),  e  stomach, 
m  primitive  kidneys  (./left  vitelline  vein,  s  cystic  vein,  a  right  vitelline  artery, 
>i  umbilical  artery,  u  umbilical  vein),  X  vitelline  duct,  i  rectum,  S  tail,  □  fore-leg, 
9'  hind-leg.      The  liver  is  removed.      (From  Cosh:) 

lungs.  The  primitive  kidneys  (m )  are  very  large  ;  they  fill 
the  greater  part  of  the  abdominal  cavity,  and  extend  from  the 
liver  ( f)  to  the  pelvic  gut.  Thus  at  the  end  of  the  first  month 
all  the  chief  organs  are  already  outlined.  But  there  are  at 
this  stage  no  features  bv  which  the  human  embryo  materially 


/■>/••  /'.  I  I.  MEMBR.  I NES  A \D  C  '/HI  Y  7..-I  TION  37 1 

differs  from  that  oi  the  dog,  the  hare,  the  ox,  or  the  horse — 
in  a  word,  oi  any  other  higher  mammal.  All  these  embryos 
have  the  same,  or  at  least  a  very  similar,  form  ;  they  can  at 
the  most  be  distinguished  from  the  human  embryo  by  the 
total  size  of  the  body  or  some  other  insignificant  difference  in 
si/e.  Thus,  for  instance,  in  man  the  head  is  larger  in  pro- 
portion to  the  trunk  than  in  the  ox.  The  tail  is  rather  longer 
in  the  dog  than  in  man.  These  are  all  negligible  differences. 
On  the  other  hand,  the  whole  internal  organisation  and  the 
form  and  arrangement  of  the  various  organs  are  essentially 
the  same  in  the  human  embryo  of  four  weeks  as  in  the 
embryos  of  the  other  mammals  at  corresponding  stages. 

It  is  otherwise  in  the  second  month  of  human  develop- 
ment. Fig.  1  l>  1  represents  a  human  embryo  of  six  weeks 
(VI.),  one  of  seven  weeks  (VII.),  and  one  of  eight  weeks 
(VI 1 1.)  at  natural  size.  The  differences  which  mark  oi'\  the 
human  embryo  from  that  of  the  dog  and  the  lower  mammals 
now  begin  to  be  more  pronounced.  We  can  see  important 
difference;,  at  the  sixth,  and  still  more  at  the  eighth,  week, 
especially  in  the  formation  of  the  head  (Plate  XIII.,  Fig. 
Mill,  etc.).  The  size  of  the  various  sections  of  the  brain 
is  greater  in  man,  and  the  tail  is  shorter.  Other  differences 
between  man  and  the  lower  mammals  are  found  in  the  relative 
si/e  of  the  internal  organs.  But  even  at  this  stage  the 
human  embryo  differs  very  little  from  that  of  the  nearest 
related  mammals,  the  apes,  especially  the  anthropomorphic 
apes.  The  features  by  means  of  which  we  distinguish 
between  them  are  not  clear  until  later  on.  Even  at  a  much 
more  advanced  stage  of  development,  when  we  can  distinguish 
the  human  foetus  from  that  o(  the  ungulates  at  a  glance,  it 
still  closely  resembles  that  of  the  higher  apes.  At  last  we 
get  the  distinctive  features,  and  we  can  distinguish  the  human 
embryo  confidently  at  the  first  glance  from  that  of  all  other 
mammals  during  the  last  four  months  of  foetal  life — from  the 
sixtli  to  the  ninth  month  of  pregnancy.  Then  we  begin  to 
find  also  the  differences  between  the  various  races  of  men, 
especially  in  regard  to  the  formation  of  the  skull  and  the  face. 
(Cf.  Chapter  XXIII.) 


FCETAL  MEMBRANES  AXD  CIRCULATION 


The  striking  resemblance  that  persists  so  long  between 
the  embryo  of  man  and  of  the  higher  apes  disappears  much 
earlier  in  the  lower  apes.  It  naturally  remains  longest  in  the 
large  anthropomorphic  apes  (gorilla,  chimpanzee,  orang,  and 
gibbon).  The  physiognomic  similarity  of  these  animals, 
which  we  find  so  great  in  their  earlier  years,  lessens  with  the 
increase  of  age.  On  the  other  hand,  it  remains  throughout 
life  in  the  remarkable  long-nosed  ape  of  Borneo  (nasalis 
lai~vatus,  Plate  XXV.).  Its  finely-shaped  nose  would  be 
regarded  with  envy  by  many  a  man  who  has  too  little  of  that 
organ.  ■  If  we  compare  the  face  of  the  long-nosed  ape  with 
that  of  abnormally  ape-like  human  beings  (such  as  the  famous 
Miss  Julia  Pastrana,  Fig.  198),  it  will  be 
admitted  to  represent  a  higher  stage  of 
development.  There  are  still  people 
among  us  who  look  especially  to  the 
face  for  the  "image  of  God  in  man." 
The  long-nosed  ape  would  have  more 
claim  to  this  than  some  of  the  stumpy- 
nosed  human  individuals  one  meets. 

This  progressive   divergence  of  the 
Fig.  198.—  Theheadof     ,  .  -  .   c  ,  .  ,    . 

Miss  Julia  Pastrana,      human  from  the  animal  form,  which  is 

JKMte)  phot°Krsiph  by     based    on    the  law   of    the   ontogenetic 

connection    between    related    forms,    is 

found    in    the    structure   of   the    internal    organs    as    well    as 

in   external  form.      It   is  also   expressed    in   the  construction 

of    the  envelopes    and    appendages    that  we    find    externally 

to  the  foetus,  and  that  we  will  now  consider  more   closely. 

Two  of  these  appendages — the  amnion  and  the  allantois — 

are  only  found  in   the   three    higher  classes  of   vertebrates, 

while   the    third,    the    yelk-sac,    is    found    in    most   of    the 

vertebrates.     This  is  a  circumstance  of  great  importance,  and 

it  gives  us  valuable  data  for  constructing  man's  genealogical 

tree. 

As  regards  the  external  membrane  that  encloses  the  ovum 

in  the  mammal  womb,  we  find  it  just  the  same  in  man  as  in 

the  higher  mammals.     The  ovum  is,  you  will  remember,  first 

surrounded  bv  the  transparent  structureless  ovolemma  or  zona 


FCET.  I  /   MEMBR.  I  .VAN  .  I ND  ( '/A7  7  7.  I  TION  373 

pellucida  (Figs,  i,  14).  But  very  soon,  even  in  the  first  week 
of  development,   it   is  replaced    by  the    permanent   chorion. 

This  arises  from  the  external  layer  o(  the  amnion,  the 
sero/emma,  or  "  serous  membrane,"  the  formation  ot  which 

we  shall  consider  presently;  it  surrounds  the  foetus  and  its 
appendages  as  a  broad,  completely-closed  sac ;  the  space 
between  the  two,  tilled  with  clear  watery  fluid,  is  the  scro- 
ccelom,  or  interamniotic  cavity  ("extra-embryonic  body- 
cavity").  But  the  smooth  surface  of  the  sac  is  quickly 
covered  with  numbers  o\  tiny  tufts,  which  are  reallv  hollow 
out-growths  like  the  fingers  of  a  glove  (Figs.  199,  204, 
217  (//:).  They  ramify  and  push  into  the  corresponding- 
depressions  that  are  formed  by  the  tubular  glands  of  the 
mucous  membrane  o(  the  maternal  womb.  Thus,  the  ovum 
secures  its  permanent  seat  (Figs.   199-207). 

In  human  ova  of  eight  to  twelve  days  this  external  mem- 
brane, the  chorion,  is  already  covered  with  small  tufts  or 
villi,  and  forms  a  ball  or  spheroid  of  six  to  eight  millimetres 
in  diameter  (Figs.  199-201).  As  a  large  quantity  of  fluid 
gathers  inside  it,  the  chorion  expands  more  and  more,  so  that 
the  embryo  only  occupies  a  small  part  of  the  space  within  the 
vesicle.  The  villi  of  the  chorion  grow  larger  and  more 
numerous.  They  branch  out  more  and  more.  At  first 
the  villi  cover  the  whole  surface,  but  they  afterwards  dis- 
appear from  the  greater  part  of  it  ;  they  then  develop  with 
proportionately  greater  vigour  at  a  spot  where  the  placenta  is 
formed  from  the  allantois. 

When  we  open  the  chorion  of  a  human  embrvo  of  three 
weeks,  we  find  on  the  ventral  side  of  the  foetus  a  large 
round  sac,  filled  with  fluid.  This  is  the  yelk-sac,  or 
"umbilical  vesicle,"  the  origin  of  which  we  have  con- 
sidered previously.  The  larger  the  embrvo  becomes  the 
smaller  we  find  the  yelk-sac.  Afterwards  we  find  the 
remainder  of  it  in  the  shape  of  a  small  pear-shaped  vesicle, 
fastened  to  a  long  thin  stalk  (or  pedicle),  and  hanging  from 
the  open  belly  of  the  foetus  (Fig.  207).  This  pedicle  is  the 
vitelline  duct,  and  is  separated  from  the  body  at  the  closing 
of  the  navel.      The  wall  of  the  umbilical  vesicle  consists,  you 


FCETAL  MEMBRANES  AND  CIRCULATION 


will  remember,  of  an  inner  plate,  the  gut-gland  layer  and  an 
outer  plate,  the  gut-fibre  layer.  It  is  therefore  made  up  of 
the  same  constituents  as  the  gut-wall  itself,  and  really  forms 
a  direct  continuation  of  it.  In  birdsand  reptiles,  in  which  the 
yelk-sac  is  much  larger,  it  contains  a  considerable  quantity 
of    nutritive    material,     albuminous     and    fatty    substances. 


Fig. 


Fig.  203. 


Fig.  199.— Human  OVUm  of  twelve  to  thirteen  days  (?).  (From  Allen 
Thomson.)  1.  Not  opened,  natural  size.  2.  Opened  and  magnified.  Within 
the  outer  chorion  the  tiny  curved  foetus  lies  on  the  large  embryonic  vesicle,  to 
the  left  above. 

Fig.  200.— Human  ovum  of  ten  days.  (From  Allen  Thomson.)  Natural 
size,  opened  ;  the  small  foetus  in  the  right  half,  above. 

Fig.  201.  — Human  foetUS  of  ten  days,  taken  from  the  preceding  ovum, 
magnified  ten  times,  a  yelk-sac,  b  neck  (the  medullary  groove  already  closed), 
c  head  (with  open  medullary  groove),  d  hind  part  (with  open  medullary  groove), 
e  a  shred  of  the  amnion. 

Fig.  202.— Human  OVUm  of  twenty  to  twenty-two  days.  (From  Allen 
Thomson.)  Natural  size,  opened.  The  chorion  forms  a  spacious  vesicle,  to 
the  inner  wall  of  which  the  small  fetus  (to  the  right  above)  is  attached  by  a 
short  umbilical  cord. 

Fig.  203.— Human  foetUS  of  twenty  to  twenty-two  days,  taken  from  the 
preceding  ovum,  magnified,  a  amnion,  b  yelk-sac,  c  lower-jaw  process  of  the 
first  gill-arch,  d  upper-jaw  process  of  same,  e  second  gill-arch  (two  smaller  ones 
behind).  Three  gill-clefts  are  clearly  seen,  /'rudimentary  fore-leg,  P" auditory 
vesicle,  /;  eye,  i  heart. 


FCETAL  MEMBRANES  AND  CIRCULATION 


These  pass  by  the  vitelline  duct  into  the  visceral  cavity,  and 
serve  as  food,  as  in  the  oviparous  monotremes.  In  the  other 
(viviparous)  mammals  the  yelk-sac    is  much   less  important 

for  the  nutrition  of  the  embryo,  and  it  atrophies  at  an  early 
stage. 


Fig.  204.—  Human  embryo  of  sixteen  to  eighteen  days.  (From  Coste. ) 
Magnified.  The  embryo  is  surrounded  by  the  amnion  ( » ),  lies  Free  with  this 
in  tin-  opened  embryonic  vesicle.  The  belly  is  drawn  up  by  the  large  yelk-sac 
( (I ),  and  fastened  to  the  inner  wall  of  the  embryonic  membrane  by  the  short 
and  thick  pedicle  (b).  Hence  the  normal  convex  curve  of  the  back  (Fig.  20.^) 
is  here  changed  into  an  abnormal  concave  surface.  //  heart,  hi  parietal 
mesoderm.  The  spots  on  the  outer  «.ill  ol  the  serolemma  are  the  roots  of  the 
branching  chorion-villi,  which  are  free  at  the  border. 

Behind  the  yelk-sac  a  second  appendage,  of  much  greater 
importance,  is  formed  at  an  early  stage  at  the  belly  oi  the 
mammal  embryo.  This  is  the  allantois  or"  primitive  u ni nary 
sac,"  an  important  embryonic  organ,  only  found  in  the  three 


376 


FCETAL  MEMBRANES  AXD  CIRCCLATIOX 


higher  classes  of  vertebrates.  In  all  the  amniotes  the 
allantois  quickly  appears  at  the  hinder  end  of  the  alimentary 
canal,  growing  out  of  the  cavity  of  the  pelvic  gut  (Fig.  208, 
;-,  //,  Fig.  209  ALC '). 


Umbilical 
vesicle 

(yelk-sac  I 

Umbilical 

cord       

(pedicle) 


Fig.   205.  -Human  embryo  of  the  fourth  week,  seven  and  a-half  mm.  long-, 
lying  in  the  dissected  chorion. 

The  allantois  originated  as  a  prolongation  of  the  urinary 
bladder  of  the  amphibia  ;  in   their  descendants,  the  protam- 

niotes  (the  ancestors  of 
the  amniotes),  it  has 
grown  out  of  the 
ccelom  of  the  embryo, 
and  has  henceforth  to 
take  a  part  in  its  nutri- 
tion. The  first  trace 
of  it  is  a  small  vesicle 
at  the  edge  of  the  cavity 
of  the  pelvic  gut ;  it 
represents  a  fold  of  the 
gut,  and  has  (like  the 
yelk-sac)  a  two-layered 
' ..     *  wall.       The    cavity    o\ 

Fig.  ,06. -Human  embryo  of  the  fourth     the    vesicle    is    clothed 

week,  with    its  membranes,  like   Fig-.  205,  but       with       the       gilt  -  gland 

a  little  older.      The  yelk-sac  is  rather  smaller, 

the  amnion  and  chorion  larger.  laver,      and      the      Outer 


FOETAL  MEMBRANES  .l\/>  CIRCULATION 


Fig.  207. -Human  embryo  with  its  membranes,  six  weeks  old.  Tin- 
outer-  envelope  of  the  whole  ovum  is  ilu-  chorion,  thickly  covered  with  its 
branching  villi,  a  product  of  the  serous  membrane.  The  embryo  is  enclosed 
in  the  delicate  amnion-sac.  The  yelk-sac  is  reduced  to  a  small  pear-shaped 
umbilical  vesicle;  its  thin  pedicle,  the  long  vitelline  duet,  is  enclosed  in  the 
umbilical  cord.  In  the  latter,  behind  the  vitelline  duet,  is  the  much  shorter 
pedicle  of  the  allantois,  the  inner  lamina  of  which  (the  gut-eland  layer)  forms 
a  large  vesicle  in  most  of  the  mammals,  while  the  outer  lamina  is  attached  to 
the  inner  wall  of  the  outer  embryonic  coat,  and  forms  the  placenta  there.  (Half 
diaerrammatii 


Fig.  jos.— Median  longitudinal  section  of  the  embryo  ofa  chick  (fifth 
day  of  incubation},  seen  from  the  right  (head  to  the  right,  tail  to  the  left). 
Dorsal   body  (episoma)  dark,  with  convex  surface,     d  gut,  0  mouth,  a  anus,  A 

liver,  g  mesentery,  /  lungs,  ;•  aurielo   of  heart,  i-  ventricle,  b   arterial   arches,  / 

aorta,  c  yelk-sac,   m  vitelline  duct,  u  allantois,   r  pedicle  of  the  allantois,  " 
amnion,  w amniotic  cavity,  s  serous  membrane,     (From  Burr.) 


.178 


FCETAL  MEMBRANES  AND  CIRCULATION 


lamina  of  the  wall  is  formed  of  the  thickened  gut-fibre  layer. 
The  little  vesicle  gets  bigger  and  bigger,  and  grows  into  a 
large  sac,  filled  with  fluid,  in  the  wall  of  which  large  blood- 
vessels are  formed.  It  soon  reaches  the  inner  wall  of  the 
foetal  cavity,  and  spreads  along  the  inner  surface  of  the 
chorion  (Fig.  209  ALC).  In  many  mammals  the  allantois 
is  so  large  that  at  last  it  surrounds  the  whole  embryo  and  the 
other  appendages  as  a  wide  membrane,  and  spreads  over  the 
whole  of  the  inner  surface  of  the  prochorion.    When  we  open 


Fig.  209.—  Diagram    of  the  embryonic  organs  of  the  mammal 

(fcetal  membranes  and  appendages).  (From  Turner.)  E,  J/,  H,  outer,  middle, 
and  inner  germ  layer  of  the  embryonic  shield,  which  is  figured  in  median 
longitudinal  section,  seen  from  the  left,  am  amnion,  AC  amniotic  cavity,  UV 
yelk-sac  or  umbilical  vesicle,  ALC  allantois,  al  periccelom  or  serocoelom  (inter- 
amniotic  cavity),  ss  serolemma  (or  serous  membrane), pc  prochorion  (with  villi). 

an  ovum  of  this  character,  we  encounter  first  a  large  cavity 
filled  with  fluid  ;  this  is  the  amniotic  cavity.  Only  when  this 
membrane  is  removed  do  we  reach  the  amniotic  vesicle  which 
encloses  the  embryo  proper. 

The  further  development  of  the  allantois  varies  con- 
siderably in  the  three  sub-classes  of  the  mammals.  The  two 
lower  sub-classes,  monotremes  and  marsupials,  retain  the 
simpler  structure  of  their  ancestors,  the  reptiles.  The  wall  of 
the  allantois  and  the  enveloping  serolemma  remains  smooth 


I- (KIM.  MEM  UK. WIS  AND  CIRCULATION 


and  without  villi,  as  in  the  birds.  But  in  the  third  sub- 
class of  the  mammals  the  serolemma  forms,  by  invagination 
at  its  outer  surface,  a  number  of  hollow  tufts  or  villi,  from 
which  it  takes  the  name  of  the  chorion  or  mallochorion.  The 
gut-fibre  layer  o(  the  allantois,  richly  supplied  with  branches 
of  the  umbilical  vessel,  presses  into  these  serous  villi  of  the 
primary  chorion,  and  forms  the  "secondary  chorion."  lis 
embryonic  blood-vessels  are  closely  correlated  to  the 
contiguous  maternal  blood-vessels  of  the  environing  uterus, 


Fig.  210.—  Embryo  of  a  dog,  from  the  right.  ,;  first,  />  second,  <•  third,  d 
fourth  cerebral  vesicle,  e  eye,fi  auditory  vesicle,  ,j,r//  first  gill-arch  (g  lower  jaw, 
//  upper  jaw),  i  second  gill-arch,  klm  heart  { I-  right  auricle,  /  right  and  m  left 
ventricle),  >i  origin  of  aorta,  «  heart-pouch,  />  liver,  </  nut,  r  vitelline  duet,  s  yelk 
sae  (torn  away),  /  allantois  (broken  off),  u  amnion,  ;■  tore-ley,  x  hind-leg. 
(  From  Bischoff. ) 

and  thus  is  formed  the   important  nutritive  apparatus  o(  the 
embrvo  which  we  call  the  placenta. 

The  pedicle  of  the  allantois,  which  connects  the  embryo 
with  the  placenta  and  conducts  the  strong  umbilical  vessels 
from  the  former  to  the  latter,  is  covered  by  the  amnion,  and, 
with  this  amniotic  sheath  and  the  pedicle  of  the  yelk-sac, 
forms  what  is  called  the  umbilical  cord  (Fig.  212  al).  As  the 
large  and  blood-filled  vascular  network  of  the  fcetal  allantois 
attaches  itself  closelv  to  the  mucous  lining  of  the  maternal 


,l8o 


FCETAL  MEMBRANES  AXD  CIRCULATION 


womb,  and  the  partition  between  the  blood-vessels  of  mother 
and  child  becomes  much  thinner,  we  get  that  remarkable 
nutritive  apparatus  of  the  foetal  body  which  is  characteristic 
of  the  placentalia  (or  choriata).  We  shall  return  afterwards  to 
the  closer  consideration  of  this  (cf.  Chapter  XXIII.). 

In  the  various  orders  of  mammals  the  placenta  undergoes 
many  modifications,  and  these  are  in  part  of  great  phylogenetic 


Fig.  21  i.— Dog-embryo,  twenty-five  days  old,  from  the  ventral  side, 
opened  (as  Figs.  196  and  197).  Pectoral  and  abdominal  walls  are  removed. 
a  nose-pits,  b  eyes,  c  lower  jaw  (first  gill-arch),  d  second  gill-arch,  efgh  heart 
(c  right,  /'lett  auricle  ;  .if  right,  /;  left  ventricle),  /aorta  (origin),  kk  liver  (in  the 
middle  between  the  folds  the  umbilical  vein  cut  through),  /  stomach,  m  gut, 
n  yelk-sac,  o  primitive  kidneys,  />  allantois,  q  fore-leg,  r  hind-leg.  The  curved 
embryo  lias  been  straightened  out.     (From  Bisrhoff.) 

importance  and  useful  in  classification.  There  is  only  one  of 
these  that  need  be  specially  mentioned — the  important  fact 
established  by  Selenka  in  1890  that  the  distinctive  human 
placentation  is  confined  to  the  anthropoids.  In  this  most 
advanced  group  of  the  mammals  the  allantois  is  very  small, 
soon  loses  its  cavity,  and  then,  in  common  with  the  amnion, 
undergoes  certain  peculiar  changes.  The  umbilical  cord 
developes    in    this    case   from    what   is   called    the    "ventral 


FXETAL  MEMBRANES  AND  CIRCULATION 


)8i 


pedicle."  Until  very  recently  this  was  regarded  as  a  structure 
peculiar  to  man.  We  now  know  from  Selenka  that  the  much- 
discussed  ventral  pedicle  is  merely  the  pedicle  of  the  allantois, 
combined  with  the  pedicle  o\  the  amnion  and  the  rudimentary 
pedicle  oi  the  yelk-sac.  It  has  just  the  same  structure  in  the 
orang  and  gibbon  (Figs.  213-216),  and  very  probable  in  the 
chimpanzee  and  gorilla,  as  in  man  ;  it  is,  therefore,  not  a 
disproof,  but  a  striking  fresh  proof,  o(  the  blood-relationship 
of  man  and  the  anthropoid  apes. 

Hence  the 
allantois  is  in- 
terestingin  three 
ways  in  connec- 
tion with  man's 
geneal  ogi  cal 
tree  :  firstly,  be- 
cause this  ap- 
pendage is  want- 
ing in  the  lower 
classes  of  verte- 
brates, and  is 
developed  only 
in  the  three 
higher  classes 
of  the  stem,  the 
reptiles,  birds, 
and  mammals  ; 
secondly,  be- 
cause the  placenta  developes  from  the  allantois  only  in  the 
placentals,  or  the  higher  mammals  and  man,  and  not  in 
the  lower  mammals  (marsupials  and  monotremes) ;  thirdly, 
because  the  remarkable  peculiarities  of  human  placentation 
are  only  found  outside  man  in  the  anthropoid  apes,  not  in 
the  other  placentals. 

We  find  only  in  the  anthropoid  apes — the  gibbon  and 
orang  of  Asia  and  the  chimpanzee  and  gorilla  of  Africa  the 
peculiar  and  elaborate  formation  of  the  placenta  that  charac- 
terises   man  (Fig.   217).      In    this    case    there    is  at  an    early 


Fie..  212.  Diagrammatic  frontal  section  of  the 
pregnant  human  womb.  ( From  Longet.  1  The 
embryo  hang?  by  the  umbilical  cord,  which  encloses 
the  pedicle  of  the  allantois  ( al ' ).  nb  umbilical  vessel, 
am  amnion,  ch  chorion,  ds  decidua  serotina,  </;•  decidua 
vera,  dr  decidua  reflexa,  >  villi  of  the  placenta,  c  cervix 
uteri,  u  uterus. 


382  FCETAL  MEMBRANES  AXD  CIRCULATION 


Fig.  215. 
Figs.  213-215.—  Embryos  of  the  kalawet-gibbon  01  Borneo  (hyiobates 

concolorj.  Fig".  213  embryo  of  seventeen  mm.  from  head  to  buttocks,  magnified 
four  times  ;  seen  from  the  left.  Fig-.  214  the  same,  seen  from  the  front.  Fig-. 
215  embryo  of  one  hundred  mm.  from  head  to  buttocks,  three-fourths  natural 
size,  in  the  same  position  as  found  in  uterus,  with  which  it  is  still  connected  by 
the  umbilical  cord.  Only  the  dorsal  half  of  the  dissected  uterus  is  shown,  and 
the  placenta  is  attached  to  the  central  part  of  this. 


FCETAL  MEMBRANES  AND  CIRCULATION 


383 


stage  an  intimate  blending  of  the  chorion  of  the  embryo  and 
the  part  o(  the  mucous  lining  o(  the  womb  to  which  it 
attaches.  The  villi  o(  the  chorion  with  the  blood-vessels 
they  contain  grows  so  completely  into  the  tissue  of  the 
uterus,  which  is  rich  in  blood,  that  it  becomes  impossible  to 
separate  them,  and  they  form  together  a  sort  of  cake.  This 
comes  away  as  the  "after-birth  "  at  parturition  ;  at  the  same 
time  the  part  o(  the  mucous  lining  of  the  uterus  that  has 
united  inseparably  with  the  chorion  is  torn  away  ;  hence  it  is 


Fig.  ji6.—  Male  embryo  of  the  Siamang-gibbon  (hyhbates  siamanga) 
of  Sumatra,  two-thirds  natural  size:  to  the  left  the  dissected  uterus,  of  which 
only  the  dorsal  half  is  given.  The  embryo  has  been  taken  out,  and  the  limbs 
folded  together;  it  is  still  connected  by  the  umbilical  cord  with  the  centre  of 
the  circular  placenta,  which  is  attached  to  the  inside  of  the  womb.  Both  this 
embryo  and  the  preceding  (Fig.  215)  take  the  head-position  in  the  womb,  and 
ibis  is  normal  in  man  also. 

called  the  decidua  (" falling-away  membrane"),  and  also  the 
"  sieve-membrane,"  because  it  is  perforated  like  a  sieve.  We 
find  a  decidua  of  this  kind  in  most  of  the  higher  placentals  ; 
but  it  is  only  in  man  and  the  anthropoid  apes  that  it  divides 
into  three  parts — the  outer,  inner,  and  placental  decidua. 
The  external  or  true  decidua  (Fig.  212  tin.  Fig.  218^)  is  the 
part  o\  the  mucous  lining  of  the  womb  that  clothes  the  inner 
surface  o\  the  uterine  cavity  wherever  it  is  not  connected 
with     the      placenta.        The     placental     or    spongy    decidua 


3«4 


FCETAl.  MEM  BR  AXES  A. YD  CIRCULATION 


( placcntalis  or  serotina,  Fig.  212  ds,  Fig.  218  d )  is  really 
the  placenta  itself,  or  the  maternal  part  of  it  (placenta 
uterina ) — namely,  that  part  of  the  mucous  lining  of  the  womb 
which  unites  intimately  with  the  chorion-villi  of  the  foetal 
placenta.  The  internal  or  false  decidua  (interna  or  reflexa, 
Fig.  212  dr,  Fig.  218  f)  is  that  part  of  the  mucous  lining  of 
the  womb  which  encloses  the  remaining:  surface  of  the  ovum, 


Foetal 
placenta 


Amniotic 
cavity 


Chorion 
(lreve) 


Uterine 
cavity 


End  of  the 
decidua 


Fig.  217.— Frontal  section  of  the  pregnant  human  womb.    (From 

Turner.)  The  embryo  (a  month  old)  hang's  in  the  middle  of  the  amniotic  cavity 
by  the  ventral  pedicle  or  umbilical  cord,  which  connects  it  with  the  placenta 
(above). 

the  smooth  chorion  (chorion  Iceve),  in  the  shape  of  a  special 
thin  membrane.  The  origin  of  these  three  different  deciduous 
membranes,  in  regard  to  which  quite  erroneous  views  (still 
retained  in  their  names)  formerly  prevailed,  is  now  quite  clear; 
the  external  decidua  vera  is  the  specially  modified  and  subse- 
quently detachable  superficial  stratum  of  the  original  mucous 
lining  of  the  womb.     The  placental  decidua  serotina  is  that 


/■</•; r. i /.  .i/am//»'A'. i x/:s  . i x/>  c yav v v.. / nox 


part  of  the  preceding  which  is  completely  transformed  by  the 
ingrowth  o(  the  chorion-villi,  and  is  used  for  constructing  the 
placenta.  The  inner  decidua  reflexa  is  formed  by  the  rise  of 
a  circular  fold  of  the  mucous  lining  (at  the  border  of  the 
decidua  vera  and  serotina),  which  grows  over  the  foetus 
(like  the  amnion)  to  the  end. 


Fig.  218.— Human  foetus,  twelve  weeks  old,  with  its  membranes, 
natural  size.  The  umbilical  cord  goes  from  its  navel  to  tin-  placenta,  b amnion, 
^chorion,  d placenta]  d'  relics  or  \illi  on  smooth  chorion]  /'  internal  or  reflex 
deciduat  g  external  or  true  decidua.     (From  B.  Schultze. ) 

The  peculiar  anatomic  features  that  characterise  the 
human  foetal  membranes  are  found  in  just  the  same  way  in 
the  higher  apes.  The  lower  apes  and  the  other  disco- 
placentals  show  more  or  less  considerable  variations,  and,  in 
general,  simpler  features.  This  applies  especially  to  the 
delicate  structure  of  the  placenta  itself,  the  blending  of  the 
chorion-villi  with  the  decidua  serotina.  The  mature  human 
placenta  is  a  circular  (less  frequently  oval)  disk  of  a  soft, 
spongy  texture,  six  to  eight  inches  in   diameter,  aboui    cue 


FCETAL  MEMBRANES  AXD  CIRCULATION 


inch  thick,  and  one  to  one  and  a  half  pounds  in  weight.  Its 
convex  outer  surface  (uniting  with  the  uterus)  is  very  uneven 
and  tufted.  Its  concave  inner  surface  (facing  the  uterine 
cavity)  is  quite  smooth,  and  covered  by  the  amnion.  As  a 
rule,  the  umbilical  cord  (funiculus  umbilicalis )  starts  from 
about  the  middle  of  the  placenta;  we  have  considered  the 
origin  of  this  from  the  ventral  pedicle.  This  also  is  covered 
or  sheathed  by  the  amnion,  which  passes  directly  into  the 
abdominal  skin  at  the  navel  end  of  the  cord  (Fig.  218).  The 
mature  umbilical  cord  is  a  cylindrical  string,  twisted  spirally 


Fig.  219.— Mature  human  foetus  (at  the  end  ot  pregnancy,  in  its  natural 
position,  taken  out  of  the  uterine  cavity).  On  the  inner  surface  of  the  latter 
(to  the  left)  is  the  placenta,  which  is  connected  by  the  umbilical  cord  with  the 
child's  navel.     (From  Bernhard Schultze.) 

on  its  axis,  generally  about  twenty  inches  long  and  half  an 
inch  thick.  It  consists  of  a  gelatinous  connective  tissue  (the 
"  Whartonian  jelly  "),  in  which  we  find  the  remainder  of  the 
vitelline  vessels  and  the  large  umbilical  vessels — the  two 
umbilical  arteries  which  conduct  the  blood  of  the  embryo  to 
the  placenta  and  the  strong  umbilical  vein  that  conveys  the 
blood  from  the  latter  to  the  heart.  The  countless  fine 
branchlets  of  this  foetal  umbilical  vessel  enter  the  ramified 
chorion-villi  of  the  foetal  placenta,  and  finally  join  in  a 
peculiar  way  with  these  to  form  the  wide  blood-filled  cavities 
that  expand  in  the  uterine  placenta  and  contain  the  maternal 


FOSTAL  MEMBRANES  AND  CIRCULATION 


387 


blood.  The  very  complicated  and  difficult  anatomic  relations 
that  develop  here  between  the  foetal  and  maternal  placenta 
are  found  in  this  form  only  in  man  and  the  anthropoid  ape; 
they  differ  more  or  less  considerably  in  all  the  other 
deciduates.  The  umbilical  cord  is  also  proportionately 
longer  in  man  and  the  apes  than  in  the  other  mammals. 


1  Pancreas  and 

/  ""'   "     portal  vein 


Inner  mouth 

of  the  womb 

(greatly 

distended)... 

Bladder 


Fig.  j.'o.  Median  section  of  the  lower  half  of  the  trunk  of  a  woman 
in  advanced  pregnancy.  The  head  of  the  child  is  already  in  the  pelvis  in, 
ili.-  normal  head-position.  The  foetal  vesicle  (the  size  of  an  apple)  is  siill  whole 
in  (In-  vagina  :  the  foetal  water  lias  not  yel  escaped.     1  From  Braune.) 

Until  recently  it  was  thought  that  the  human  embryo  was 
distinguished  by  its  peculiar  construction  of  a  solid  allantois 
and  a  special  ventral  pedicle,  and  that  the  umbilical  cord 
developed  from  this  in  a  different  way  from  in  the  other 
mammals.  The  opponents  of  the  unwelcome  "  ape-theory  " 
laid  great  stress  on  this,  and  thought  they  had  at  last 
discovered  an   important  indication    that  separated    man  from 


FCETAL  MEMBRAXES  AND  CIRCULATIOX 


all  the  other  placentals.  But  the  remarkable  discoveries 
published  by  the  distinguished  zoologist  Selenka  in  1890 
proved  that  man  shares  these  peculiarities  of  placentation 
with  the  anthropoid  apes,  though  they  are  not  found  in  the 
other  apes.  Thus  the  very  feature  which  was  advanced  by 
our  critics  as  a  disproof  became  a  most  important  piece  of 
evidence  in  favour  of  our  pithecoid  origin. 

The  new  facts  that  Selenka  discovered  during  his  investi- 
gation of  this  question  in  India  are  so  important,  and  yield 
such  far-reaching  conclusions,  that  I  will  give  the  results  in 
his  own  words  : — 

Some  embryonic  organs  are  developed  earlier  and  some  later  in  the  apes 
and  man  than  in  the  other  mammals.  Among  the  anticipated  structures  are: 
( 1)  the  innumerable  chorion-villi,  (2)  the  coelom-sacs,  by  the  expansion  of  which 
the  yelk-sac  is  early  removed  and  the  amnion  closed,  and  (3)  the  pedicle  of  the 
allantois.  On  the  other  hand,  we  have  the  following-  retarded  structures:  (1) 
the  yelk-sac.  It  is  true  that  it  quickly  separates  from  the  wall  of  the  embryonic 
vesicle,  but  its  vascular  network  only  developes  later  on.  As  it  has  completely 
lost  its  earlier  function  of  respiratory  and  nutritive  organ,  it  must  be  regarded 
as  a  rudimentary  organ.  It  sends  no  vessels  into  the  chorion,  all  the  blood- 
vessels of  which  are  exclusively  allantoic.  (2)  The  rise  of  the  allantoic  cavity 
also  is  delayed,  and  (3)  the  differentiation  of  the  germinative  area.  As  special 
structures  we  may  designate:  (1)  the  looser  texture  of  the  somatopleura, 
which  lines  the  chorion  ;  (2)  the  persistence  of  the  pedicle  of  the  allantois  ;  (3) 
the  expansion  of  the  amnion  and  its  blending  with  the  chorion  ;  (4)  the  forma- 
tion of  two  placenta;  side  by  side,  one  of  which  may  remain  rudimentary;  (5) 
the  degeneration  of  the  yelk-sac  into  a  rudimentary  organ  ;  and  (6)  the  attach- 
ment of  the  non-placental  part  of  the  foetal  membrane — whether  it  be  the 
chorion  laeve  or  the  decidua  reflexa — to  the  surrounding  wall  of  the  uterus. 

A  third  embryonic  appendage,  which  we  have  already 
mentioned — the  amnion  or  "water-membrane" — is  also,  like 
the  allantois,  one  of  the  characteristic  features  of  the  three 
higher  classes  of  vertebrates.  We  have  introduced  the 
amnion  when  dealing  with  the  severance  of  the  embryo  from 
the  embryonic  vesicle  (p.  308).  We  found  that  its  walls  rise 
about  the  embryonic  body  in  the  form  of  a  circular  fold.  In 
front  this  fold  rises  to  some  height  in  what  is  called  the  hood 
or  sheath  of  the  head  (Fig.  222  ks);  behind  also  it  curves 
over  considerably  as  the  hood  or  sheath  of  the  tail  fss);  to 
the  right  and  left  the  fold  is  at  first  lower,  and  is  known  as 
the  side-hood  or  sheath  (Fig.  226).  All  these  "  hoods  "  or 
"  sheaths  "  are  merely  portions  of  a  continuous  circular  fold 


Fig.  225. 
Figs.    221-225.    Five   diagrammatic  longitudinal   sections   of  the 
maturing  mammal  embryo  and  its  envelopes.    In  Kitr^-  221-224  ,lu' 

longitudinal  section  goes  through  the  sagittal  or  middle  plane  of  the  body, 
which  cuts  it  into  right  and  left  halves  1  in  li.y.  225  the  foetus  is  seen  from  the 
left.  In  Fig.  221  the  prochorion  I ' </ ).  dotted  with  villi  (d'J,  encloses  the 
embryonic  vesicle,  the  wall  oi  which  consists  of  the  two  primary  germinal 
layers.  Between  the  outer  (a)  and  inner-  ( i )  germinal  layer  the  middle 
layer  ( m)  has  developed  in  the  region  of  the  germinative  area.  In  Fig. 
222  the  embryo  ( ,■ )  begins  to  separate  from  the  embryonic  vesicle  fds), 
while  the  wall  of  the  amniotic  fold  rises  round  it  (in  front  as  head-sheath,  is , 
behind  as  tail-sheath,  ss).  In  Fig.  221,  the  edges  of  the  amniotic  fold  ( ttm ) 
meet  over  the  back  of  the  embryo,  and  thus  form  the  amniotic  cavity  (ah); 
the  embryo  < e  J  separating  still  more  from  the  embryonic  vesicle  fils),  the 
alimentary  canal  (do)  is  formed,  the  allantois  (al)  growing  out  of  its  hinder 
end.      In   Fig.    224   the   allantois   <"<;/>  is   larger,  the  yelk-sac  (ds)  smaller.      In 


FCETAL  MEMBRAXES  AXD  CIRCVLATIOX 


that  runs  round  the  embryo.  It  grows  higher  and  higher, 
rises  up  like  a  rampart,  and  at  last  curves  like  a  grotto  over 
the  body  of  the  embryo.  The  edges  of  the  circular  fold  touch 
and  join  (Fig.  227).  Thus  in  the  end  the  embryo  is  enclosed 
in  a  membranous  sac,  which  is  filled  with  the  amniotic  fluid 
(Figs.  224,  225  ah). 

When  the  sac  is  completely  closed,  the  inner  plate  of  the 
fold,  which  forms  the  real  wall  of  the  amniotic  sac,  separates 
altogether  from  the  outer.  The  latter  attaches  itself  internally 
to  the  prochorion,  replaces  it,  and  becomes  itself  the 
permanent  outer  envelope  of  the  embryo,  described  by  Baer 
as  the  "  serous  membrane."  This  serolemma  consists,  like 
the  thin  wall  of  the  amnion-sac,  of  two  layers — the  neural  and 
the  parietal  germ-layers.  The  latter  is  in  this  case  very  thin 
and  delicate,  but  can  easily  be  recognised  as  a  direct  continua- 
tion of  the  skin-fibre  layer.  Naturally,  in  harmony  with  the 
folding  process,  the  parietal  middle  layer  is  turned  inwards 
in  the  serolemma  and  outwards  in  the  amnion.  The  space 
between  it  and  the  allantois  is  the  periccelom  or  the  inter- 
amniotic  cavity  (the  extra-embryonic  body-cavity,  Fig.  209  al). 

The  phylogenetic  cause  of  this  ontogenetic  formation  of 
the  amnion  is  to  be  sought  on  mechanical  lines  in  the  fact 
that  the  body  of  the  embryo  has  gradually  sunk  into  the 
underlying  yelk-sac,  thus  leaving  a  circular  fold  of  membrane 
around  it.  The  growth  of  the  latter  into  a  completely  closed 
sac,  filled  with  fluid,  is  explained  on  the  theory  of  selection 
by  the  great  service  which  so  admirable  a  protective  structure 
offers  to  the  delicate  embryo. 

Of  the  three  vesicular  appendages  of  the  amniote  embryo 
which  we  have  now  described  the  amnion  has  no  blood- 
vessels at  any  moment  of  its  existence.     But  the  other  two 

Fig-.  225  the  embryo  already  shows  the  gill-clefts  and  the  rudiments  of  the  two 
pairs  of  legs  ;  the  chorion  has  branched  villi.  In  all  five  figures  :  e  embryo, 
a  outer  germinal  layer,  m  middle  germinal  layer,  i  inner  germinal  layer, 
am  amnion  (is  head  sheath,  ss  tail  sheath),  ah  amniotic  cavity,  as  amniotic  sheath 
of  the  umbilical  cord,  kh  embryonic  vesicle,  i/s  yelk-sac  (umbilical  vesicle),  dg- 
vitelline  duct,  df  gut-fibre  layer,  dd  gut-gland  layer,  al  allantois,  vl=hh  place 
of  heart,  d  ovolemma  or  prochorian,  d'  villi  of  same,  sh  serous  membrane 
(serolemma),  sz  villi  of  same,  ch  chorion,  chs  villi  of  same,  st  terminal  vein, 
/  periccelom  or  seroccelom  (the  space  between  the  amnion  and  chorion,  filled 
with  fluid).      (From  Kullikcr.)     Cf.  Plate  VII.,  Figs.   14  and  15. 


FOETAL  MEMBRANES  AND  CIRCULATION 


vesicles,  the  yelk-sac  and    the  allantois,  are   equipped   with 
large  blood-vessels,  and  these  effect  the  nourishment  of  the 

embryonic     body. 

We    may   take  the 

opportunity       to 

make  a  few  general 

observations  on  the       '  "■  --'••    Transverse  section  of  the  embryo 

of  a  chick  (a  little  behind  the  anterior  opening' of  the 

first    Circulation     in     gut)  at  the  end  ofthe  first  day  of  incubation.    Above 

rl-io  wnhrvn   -irul  its     ls  ""'  medullary  groove,  below  the  gut-groove,  still 

tiic  em  Dry  o  ana  its    wkl>.  open    0n  each  side  we  siv  the  oullim.  ol  tlu. 

central    or  "an     the  body-cavity  between  the  skin-fibre  layer  and  the  gut- 

6      '   _  fibre  layer.     To  the  right  and  left  of  it  outwards  the 

heart.        The      first  lateral  hoods  ofthe  amnion  are  beginning  lo  rise. 

,,        ,               .          .  ( From  Remak.) 

blood-vessels,    the 

heart,  and  the  first  blood   itself,  are  formed  from   the   gut-fibre 
layer.      Hence    it    was   called    by   earlier   embryologists    the 

"  vascular     layer." 


In  a  sense  the  term 
is  quite  correct. 
But  it  must  not  be 
understood  as  if  all 
the  blood-vessels  in 
the  body  came  from 
this  layer,  or  as  if 
the  whole  of  this 
laser  were  taken  up 
only  with  the  for- 
mation of  blood- 
\  essels.  Neither 
of  these  supposi- 
tions is  true. 
Blood-vessels   may 

Fig.  227.    Transverse  section  of  the  embryo  be  formed  indepen- 

ofa  chick  in  the  region  of  the  navel  (of  the  fifth  day  ,        ,  _       ■  ,*!,_- 

of  incubation).     The  amniotic    folds  (am)  almost  dentlj        in       othe. 

touch  above  over  the  back  of  the  embryo.     Thegut  n^pts,  especially    in 

fdj,   -.till   open,    passes    below   into   the    yelk-sac.  r         >       1 

(//"  sfut-iihn-  layer,  sA  chorda,  sa  aorta,  vc  cardinal  the      various      pro- 
veins,  bh  ventral  wall,  nol  yet  closed,  v  fore,  g  hind  -     , 

roots  of  spinal  nerves,  mii  muscle-plate,   lip  cutis-  ducts  Ot    the    SKin- 

plate,  A  horny-plate.     (From  Remai.)  fibre     layer.        The 

tissue    that    composes    the    blood-vessels    belongs    to    those 
secondary  products  oi  the  mesoderm  that  do   not  divide  as 


FCETAL  MEMBRANES  AND  CIRCULATION 


epithelial  plates,  but  may  arise  anywhere  in  holes  between 
the  epithelial  products  of  the  germ-layers,  and  were  marked 
off  by  Hertwig  under  the  title  of  intermediate  layer  or 
mesenchyma.  However,  according  to  some  observers,  the 
inner  vascular  epithelium  originates  from  the  entoderm. 

The  heart  and  the  blood-vessels  and  the  vascular  system 
generally  are  by  no  means  among  the  oldest  parts  of  the 
animal  organism.     Aristotle  believed  that  the  heart  was  one 

of  the  first  organs  to  be 
formed  in  the  chicken  ; 
and  many  later  writers 
adopted  this  opinion. 
But  this  is  not  at  all 
the  case.  The  chief 
parts  of  the  body — the 
four  secondary  germ- 
layers,  the  medullary 
tube  and  chorda  —  are 
formed  long  before  there 
is  any  trace  of  the  vas- 
cular system.  As  we 
shall  see  later,  this  fact 
is  in  complete  harmony 
with  the  phylogeny  of 
the  animal  kingdom. 
The  ccelenteria  (gas- 
trasads,  sponges,  cni- 
daria,  and  platodes),  to 
which  a  section  of  our  earliest  animal  ancestors  belonged, 
have  neither  blood  nor  heart.  The  vermalia  were  developed 
at  a  comparatively  late  date  from  these  bloodless  ccelenteria, 
and  the  higher  vermalia  in  which  a  vascular  system  of  the 
simplest  form  developes  (frontonia  )  later  still  from  the 
non-vascular  lower  vermalia  (rotatoria);  from  the  higher 
vermalia  are  descended  the  much  younger  vertebrates. 

The  first  blood-vessels  of  the  mammal  embryo  have  been 
considered  by  us  previously  in  the  transverse  sections  on 
Figs.  148-151   (p.  314).     They  are,  firstly,  the  two  primitive 


Fig.    228.— Transverse   section  of  the 

embryo  of  a  chick  in  the  region  of  the  shoul- 
der (of  the  fifth  day  of  incubation).  The 
section  passes  through  the  rudimentary  fore- 
leg (or  wing,  e).  The  amniotic  folds  are  joined 
over  the  back  of  the  embryo.  (From  Remak.) 
Cf.  Figs.  225,  226,  and  227;  also  Plate  VII., 
Fig.  14. 


FOETAL  MEMBRANES  AND  CIRCULATION 


arteries  or  aortas,  which  lie  in  the  narrow  longitudinal  clefts 
between  the  provcrtebne,  the  lateral  plates,  and  the  gut-gland 
layer  (Figs.  141  t/o,  14800)  ;  and,  secondly,  the  two  principal  or 
cardinal  veins,  which  appear  a  little  later,  farther  out  than  the 
former,  above  the  primitive  renal  ducts  (Figs.  140   157  cur). 

The  heart  arises  in  just  the  same 
way  and  in  connection  with  these  first 
vessels,  in  the  lower  wall  of  the  fore- 
gut,  at  the  throat,  where  the  heart 
remains  throughout  life  in  the  fish. 
The  heart  of  the  vertebrate  is  originally 
only  a  local  enlargement  of  the  median 
visceral  vessel,  which  runs  on  the  lower 
wall  of  the  gut,  and  which  we  have 
called  the  principal  vein  in  our  study 
of  the  primitive  vertebrate  (Figs.  101, 
10^  7').  The  simple,  spindle-shaped 
heart,  that  we  assume  to  have  been 
here  at  the  limit  of  the  head  and  trunk, 
is  found  at  the  same  spot,  immediately 
behind  the  gill-gut,  in  the  embryos  of 
the  acrania  and  the  cyclostoma  (Plate 

XIX.,  Fig.    16  //)  and   the  fishes.      By  Fig.    229.  —Human 

.,  ,  .  .  .,      '      embryo  of  fourteen   to 

the  contraction  01  its  muscular  wall  the    eighteen  days,  opened  on 

venous   blood    that    is  brought   by  the    |!u>  (VL'",r;;'  side-     u"der 

&  '  the  frontal   process  ot   the 

subintestinal  vein  is  driven  forward  into    lu'atl  (')  tlu-  lleart  (<~)  >* 

.  seen  in  the  cardiac  Cavity 

the  branchial  artery  (on  the  under  side    (/>),  with  the  base  o(  the 

c  ,1       1  11         »\  aorta   ( b  I.     The   yelk-sac 

ot  the  branchial  gut).  (n)  lias  bivn  ronioVl.d  for 

The  rudimentary  heart  is  single  in  tho  m;,st  part  (at   *  the 

°  inosculation   ot    the    fore- 

the    amphibia  also.      In   the   amniotes,  arm),    g  primitive  aortas 

,                      ......              .  (lying  under  the  primitive 

however,   it    is   double   trom    the    hrst,  vertebra),    >    rectum,   « 

having  two  distinct  halves  (Fig.  1  -,7  /;).     aUantois  ('"  its  P%di^<  '' 
a  \      t>      0/      /      amnion.     [From  Caste.) 

But    the    two    halves   soon    degenerate 

and  unite,  in  the  ventral  middle  line  of  the  wall  of  the 
fore-gut,  to  form  a  single  simple  tube.  The  double  structure 
is  a  later  cenogenetic  phenomenon,  mechanically  deter- 
mined by  the  flat  expansion  of  the  embryonic  shield  on  the 
large  yelk-vesicle. 


FCETAL  MEMBRAXES  AXD  CIRCULATIOX 


The  simple,  spindle-shaped  structure  of  the  heart,  which 
separates  from  the  ventral  wall  of  the  head-gut,  consists  of 
the  two  germinal  layers  of  the  gut-wall,  a  small  fold  of  the 
gut-gland  layer  being  taken  into  the  tube.  From  this  is 
formed  the  endocard,  the  epithelial  inner  cellular  lining  of  the 
heart.  Its  thick  muscular  wall,  the  myocard,  is  formed  by 
the  cells  of  the  gut-fibre  layer  or  visceral  middle  layer.  From 
this  also  come  the  red  blood-cells,  and  the  first  traces  of  the 
vessels  that  are  connected  with  the  heart.     These  also  are 

at  first  solid,  round 
strings  of  cells.  They 
are  then  hollowed  out  by 
the  secretion  of  fluid  at 
their  axis.  Some  of  the 
cells  are  detached  and 
float  in  the  fluid,  and  thus 
become  blood-cells.  This 
applies  both  to  the  arteries 
(which  convey  the  blood 
from  the  heart)  and  the 
veins  (which  convey  it  to 
the  heart).  The  white 
blood-cells  (lymph-cells 
or  leucocytes)  are  travel- 
ling cells,  originating  in 
the  mesenchyma  and 
passing  subsequently  into 
the  blood-vessels. 

The  heart  of  every 
vertebrate  lies  at  first  in 
the  ventral  wall  of  the  fore-gut,  or  in  the  ventral  (or  cardiac) 
mesentery,  by  which  it  is  connected  for  a  time  with  the  wall 
of  the  body.  But  the  heart  soon  severs  itself  from  the  place  of 
ts  origin,  and  lies  freely  in  a  cavity — the  cardiac  cavity  (Fig. 
230  c).  For  a  short  time  it  is  still  connected  with  the  former  by 
the  thin  plate  of  the  mesocardium  (hg).  Afterwards  it  lies 
quite  free  in  the  cardiac  cavity,  and  is  only  directly  connected 
with  the  gut-wall  by  the  vessels  which  issue  from  it  (Fig.  230). 


Fig.  230.— Diagrammatic  transverse 

Section  Of  the  head  of  a  mammal  em- 
bryo. /;  horny  plate,  m  medullary  tube 
(cerebral  vesicle),  tnr  wall  of  same,  /  cutis- 
plate,  s  rudimentary  skull,  c/i  chorda,  /•  gill- 
arches,  nip  muscular  plate,  c  cardiac  cavity, 
foremost  part  of  the  body-cavity  (cceloma), 
d  alimentary  canal,  dd  gut-gland  layer,  df 
visceral  muscular  plate,  hg  mesocardium, 
/iw  wall  of  heart,  hi  ventricle  of  heart,  ab 
aorta-arch,  a  section  of  aorta-stem. 


FCETAL  MEMBRANES  AND  CIRCULATION  395 

The  fore-end  of  the  spindle-shaped  tube,  winch  soon 
bends  into  an  S-shape  (Fig.  232),  divides  into  a  right  and 
left  branch.  These  tubes  are  bent  upwards  arch-wise,  and 
represent  the  first  arches  of  the  aorta.  They  rise  in  the  wall 
of  the  fore-gut,  which  they  enclose  in  a  sense,  and  then  unite 
above,  in  the  upper  wall  of  the  fore  gut-cavity,  to  form  a  large 
single   artery,  that    runs   backward    immediately   under   the 


Fig.  231.  Vitelline  vessels  in  the  germinative  area  of  a  chick- 
embryo,  at  the  close  of  the  third  day  of  incubation.  (From  Balfour.')  The 
1  germinative  area  is  seen  from  the  ventral  side  :  the  arteries  are  dark, 
tin-  wins  light.  //  In-art,  .l.l  aorta-arches,  Ao aorta,  ROf.A  right  omphalo- 
mesenteric artery,  &  '/'.  sinus  terminalis,  L.DfumX  R.OfTighX  and  left  omphalo- 
mesenteric veins, 5.  V.  sinus venosus,  ZXC  ductus  Cuvieri,  S.CaK  and  I'.Cn  fore 
and  hind  cardinal  veins. 


chorda,  and  is  called  the  aorta  (Fig.  231  Ao).  The  first  pair 
of  aorta-arches  rise  on  the  inner  wall  of  the  first  pair  oi  gill- 
arches,  and  so  lie  between  the  first  gill-arch  ( k )  and  the  fore- 
gut  (d),  just  as  we  find  them  throughout  life  in  the  fishes. 
The  single  aorta,  which  results  from  the  upper  conjunction  of 
these   two    first  vascular  arches,   divides  again   immediately 


396 


FCETAL  MEMBRAXES  AXD  CIRCVLATIOX 


into  two  parallel  branches,  which  run  backwards  on  either 
side  of  the  chorda.  These  are  the  primitive  aortas  which  we 
have  already  mentioned  ;  they  are  also  called  the  posterior 
vertebral  arteries.  These  two  arteries  now  give  off  at  each 
side,  behind,  at  right  angles,  four  or  five  branches,  and  these 
pass  from  the  embryonic  body  to  the  germinative  area  ;  they 
are    called    omphalo-mesenteric   or  vitelline   arteries.     They 

represent  the  first  rudi- 
ment of  a  foetal  circula- 
tion. Thus,  the  first 
blood-vessels  pass  over 
the  embryonic  body  and 
reach  as  far  as  the  edge 
of  the  germinative  area. 
At  first  they  are  confined 
to  the  dark  or  "  vas- 
cular "  area.  But  they 
afterwards  extend  over 
the  whole  surface  of  the 
embryonic  vesicle.  In 
the  end,  the  whole  of  the 
yelk-sac  is  covered  with 
a  vascular  net-work. 
These  vessels  have  to 
gather  food  from  thecon- 


Fig.  232.— Boat-shaped  embryo  of  the 

dog,  from  the  ventral  side,  magnified  about 
ten  times.  In  front  under  the  forehead  we 
can  see  the  first  pair  of  g*ill-arches  ;  under- 


litory 

divides  behind  into  the  two  vitelline  veins, 
which  expand  in  the  germinative  area  (which 
is  torn  off  all  round).  On  the  floor  of  the 
open  belly  lie,  between  the  protovertebrse, 
the  primitive  aortas,  from  which  five  pairs 
of  vitelline  arteries  are  t;iven  off.  (From 
Bhchoff. ) 


neath   is  the  S-shaped  heart,  at  the  sides  of      tents  of   the    velk-sac  and 
which  are  the  auditorv  vesicles.     The  heart  _         ' 

convey  it  to  the  em- 
bryonic body.  This  is 
done  by  the  veins,  which 
pass  first  from  the  ger- 
minative area,  and  after- 
wards from  the  yelk-sac,  to  the  farther  end  of  the  heart. 
They  are  called  vitelline,  or,  frequently,  omphalo- 
mesenteric, veins. 

Thus,  the  first  embryonic  circulation  (Figs.  231-234)  is 
arranged  in  the  following  simple  way  in  the  three  higher 
classes  of  vertebrates.  The  simple  tubular  heart  (Fig.  234  d) 
divides,  both  in  front  and  behind,  into  two  vessels.     The  hind 


■wws*, 


/■(/■: TAL  MEMBRANES  AND  CIRCULATION  397 

vessels  are  tlie  afferent  vitelline  veins.  They  absorb  nutritive 
matter  from  the  embryonic  vesicle  or  the  yelk-sac,  and  convey 

it  to  the  embryonic  body.  The  anterior  vessels  are  the 
efferent  branchial  arteries,  which  pass  round  the  fore  part  of 
the  gut  in  the  shape  of  the  rising  aortic-arches  ;  they  unite  to 
form  the  aorta.  The  two  branches  that  are  formed  by  the 
splitting  of  the  main  artery — the  primitive  aortas — give  o\( 
vitelline  arteries  to   right  and    left,  and   these   pass  from   the 


Fig.  j^;,.  -Embryonic  shield  and  germinative  area  of  a  hare,  in 
which  wo  see  the  first  outline  of  the  blood-vessels,  seen  from  the  ventral  side. 
magnified  about  ton  times.  The  hind  end  of  the  simple  heart  ( n  j  divides  into 
two  strong  vitelline  veins,  and  those  form  a  vascular  network  in  the  dark  area 
(which  looks  light  on  the  blaek  ground).  At  the  bead-end  we  can  see  the  fore 
brain  with  the  two  optic  vesleles  ( b.  h ).  The  darker  middle  ot  the  embryo  is 
the  wide-open  visceral  cavity.  On  each  side  of  the  chorda  wo  see  ten  proto- 
vertebrse.     (From  Bishoff.) 

body  of  the  embryo  to  the  germinative  area.  Here,  and  in 
the  periphery  of  the  umbilical  vesicle,  we  distinguish  two 
layers  of  vessels,  the  surface-layer  of  arteries  and  the  lower 
layerof  veins.  The  two  are  connected.  At  first  this  vascular 
system  only  extends  over  the  periphery  of  the  germinative 
area  to  its  border.  Here,  at  the  edge  of  the  dark  vascular 
area,  all  the  branches  unite  in  a  large  terminal  vein 
(Fig.    2,^4    a).     This    vein    disappears    later    on,    when    the 


398  FCETAL  MEMBRANES  AXD  CIRCULATION 

formation  of  vessels  proceeds  further  in  the  course  of 
development,  and  then  the  vitelline  vessels  cover  the  whole  of 
the  yelk-sac.  These  vessels  naturally  atrophy  with  the 
degeneration  of  the  umbilical  vesicle  ;  their  importance  is 
restricted  to  the  first  period  of  the  life  of  the  embryo. 

This    vitelline    circulation     is    afterwards    replaced    by    a 
second,    that    of    the    allantois.      Large     blood-vessels    are 


Fig.  ^34.— Embryonic  shield  and  germinative  area  of  a  hare,  in  which 

the  first  vascular  system  is  fully  formed,  seen  from  the  ventral  side,  magnified 
about  five  times.  The  posterior  end  of  the  S-shaped  heart  (d)  divides  into  two 
strong  vitelline  veins,  each  of  which  gives  off  a  fore  (b )  and  hind  (c)  branch. 
The  ends  of  these  unite  in  the  circular  terminal  vein  (a).  In  the  germinative 
area  we  see  the  coarser  (deeper-lying)  venous  net  and  the  finer  (more  super- 
ficial) arterial  net.  The  vitelline  arteries  ( f )  open  into  the  two  primitive  aortas 
( e ).  The  dark  area,  which  surrounds  the  head  like  an  aureole,  corresponds  to 
the  depression  of  the  head-hood.     (From  Bischoff.) 

developed  in  the  wall  of  the  urinary  sac  or  the  allantois,  as 
before,  from  the  gut-fibre  laver.  These  vessels  grow  larger 
and  larger,  and  are  very  closely  connected  with  the  vessels 
that  develop  in  the  body  of  the  embryo  itself.  Thus,  the 
secondary,  allantoic  circulation  gradually  takes  the  place  of 
the  original  vitelline  circulation.  When  the  allantois  has 
attached   itself  to   the    inner  wall   of    the   chorion  and    been 


F(I-:t.  I L  MEMBR.  I SES  .  I ND  ( '//>'(  7 '/..  1  TION  31  n  1 

converted  into  the  placenta,  its  blood-vessels  alone  effect  the 
nourishment  of  the  embryo.  They  are  called  umbilical 
\esseN,  and  are  originally  double — a  pair  of  umbilical  arteries 
and  a  pair  oi  umbilical  veins.  The  two  umbilical  veins 
(Fig.  iq6  //),  which  convey  blood  from  the  placenta  to  the 
heart,  open  at  first  into  the  united  vitelline  veins.  The  latter 
then  disappear,  and  the  right  umbilical  vein  goes  with  them, 
so  that  henceforth  a  single  large  vein,  the  left  umbilical  vein, 
conducts  all  the  blood  from  the  placenta  to  the  heart  of  the 
embryo.  The  two  arteries  of  the  allantois,  or  the  umbilical 
arteries  (Figs.  196  /i,  197  «),  are  merely  the  ultimate  termi- 
nations of  the  primitive  aortas,  which  are  stongly  developed 
afterwards.  This  umbilical  circulation  retains  its  importance 
until  the  nine  months  of  embryonic  life  are  over,  and  the 
human  embryo  enters  into  the  world  as  an  independent  indi- 
vidual. The  umbilical  cord  (Fig.  212  al),  in  which  these 
large  blood-vessels  pass  from  the  embryo  to  the  placenta, 
comes  away,  together  with  the  latter,  in  the  after-birth,  and 
with  pulmonarv  respiration  begins  an  entirely  new  form  of 
circulation,  which  is  confined  to  the  body  of  the  infant. 

There  is  a  great  phylogenetic  significance  in  the  perfect 
agreement  which  we  find  between  man  and  the  anthropoid 
apes  in  these  important  features  of  embryonic  circulation,  and 
the  special  construction  of  the  placenta  and  the  umbilical 
cord.  We  must  infer  from  it  a  close  blood-relationship  of 
man  and  the  anthropomorphic  apes,  a  common  descent  of 
them  from  one  and  the  same  extinct  group  of  lower  apes. 
Huxley's  "  pithecometra-principle  "  applies  to  these  onto- 
genetic features  as  much  as  to  any  other  morphological 
relations  :  "  The  differences  in  construction  of  any  part  of  the 
body  are  less  between  man  and  the  anthropoid  apes  than 
between  the  latter  and  the  lower  apes." 

This  important  Huxleian  law,  the  chief  consequence  of 
which  is  "  the  descent  of  man  from  the  ape,"  has  lately  been 
confirmed  in  an  interesting  and  unexpected  way  from  the  side 
of  the  experimental  physiology  of  the  blood.  The  experi- 
ments of  Hans  Friedenthal  at  Berlin  have  shown  that  human 
blood,  mixed   with  the  blood  of  lower  apes,  has  a  poisonous 


FCETAL  MEMBRANES  AXD  CIRCULATION 


effect  on  the  latter  ;   the  serum  of  the  one  destroys  the  blood- 
cells  of  the  other.     But  this  does  not  happen  when  human 


Fig.  235. — Lar  OF  White-handed  gibbon  (hylobates  lar  or  albimanus) 
from  the  Indian  main-land.      (From  Brehm.) 

blood  is  mixed  with  that  of  the  anthropoid  ape.  As  we  know 
from  many  other  experiments  that  the  mixture  of  two  different 
kinds  of  blood  is  only  possible  without  injury  in  the  case  of 


/••</•: /.]/.  MEMBRANES  AND  CIRCULATION  401 

two  closely  related  animals  of  the  same  family,  we  have 
another  proof  o\  the  close  blood-relationship,  in  the  literal 
sense  of  the  word,  of  man  and  the  anthropoid  ape. 


FIG.  236.— Young  orang  fsafyrus  orang),  asleep. 

The  existing  anthropoid  apes  are  only  a  small  remnant  of 
a  large  family  of  eastern  apes  (or  catarrhtnce),  from  which 
man  was  evolved  about  the  end  of  the  tertiary  period.  They 
fall     into    two   geographical    groups — the    Asiatic    and    the 

.21) 


FCETAL  MEMBRANES  AND  CIRCULATION 


African  anthropoids.  In  each  group  we  can  distinguish  two 
genera.  The  oldest  of  these  four  genera  is  the  gibbon 
( hylobates,  Fig.  235);  there  are  from  eight  to  twelve  species 
of  it  in  the  East  Indies.  I  made  observations  of  four  of  them 
during  my  voyage  in  the  East  Indies  (1901),  and  had  a 
specimen  of  the  ash-grey  gibbon  (hylobates  leuciscus  )  living 


Fig.  237. — Wild  Orang  ( dyssatyrus  auritus).    (From  R.  Fick  and  Leutemann. ) 

for  several  months  in  the  garden  of  my  house  in  Java.  I 
have  described  the  interesting  habits  of  this  ape  (regarded  by 
the  Malays  as  the  wild  descendant  of  men  who  had  lost  their 
way)  in  my  Malayischen  Reisebriefen  (chap.  xi.).  Psycho- 
logically, he  showed  a  good  deal  of  resemblance  to  the 
children  of  my  Malay  hosts,  with  whom  he  played  and 
formed  a  very  close  friendship. 


POSTAL  MEMBRANES  AND  CIRCULATION 


403 


The  second,  larger  and  stronger,  genus  o(  Asiatic  anthro- 
poid ape  is  the  orang  (satyrus);  he  is  now  found  only  in  the 
islands  o(  Borneo  and  Sumatra.  Selenka,  who  lias  lately 
published  a  very  thorough  Study  of  the  Development  and 
Cranial  Structure  of  Hie  .  Anthropoid  Apes  (1899),  distinguishes 
ten  races  of  the  orang,  which  may,  however,  also  be  regarded 


<& 


Fig.  238.— Head  of  an  old  male  orang-utang  (satyrus  orang),  without 
cheek-pads.     I  From  Brehm.  | 

as  "  local  varieties  or  species."  They  tall  into  two  sub-genera 
or  genera  :  one  group,  dissatyrus  (orang-bentang,  Fig.  237), 
is  distinguished  for  the  strength  of  its  limbs,  and  the  forma- 
tion of  very  peculiar  and  salient  cheek-pads  in  the  elderly 
male;  these  are  wanting  in  the  other  group,  the  ordinary 
orang-outang  (eusatyrus,  Figs.  236,  238). 


4o4 


FOETAL  MEMBRANES  AND  CIRCULATION 


Several  species  have  lately  been  distinguished  in  the  two 
genera  of  the  black  African  anthropoid  apes  (chimpanzee  and 
gorilla).     In  the  genus   anthropithecus  (or  anthropopithecus, 


Fig.    239.— The   bald-headed   chimpanzee   (anthropithecus  calvusj. 

Female.  This  fresh  species,  described  by  Frank  Beddard  in  1897  as  troglodytes 
ralvus,  differs  considerably  from  the  ordinary^.  niger(Fig.  240)  in  the  structure 
of  the  head,  the  colouring,  and  the  absence  of  hair  in  parts. 


/■'</  TAL  membranes  and  circulation 


formerly  troglodytes)  the  bald-headed  chimpanzee,  A.  calvus 
(Fig.  230),  and  the  gorilla-like  .1.  mafuca  (Fig.  -241)  differ 
very  strikingly  from  the  ordinary  antkropithecus  niger 
(Fig.  240),  not  only  in  the  size  and  proportion  of  many  parts 
of  the  body,  but  also  in  the  peculiar  shape  of  the  head, 
especially  the  ears  and  lips,  and  in  the  hair  and  colour.     The 


Fig.  240.—  Female  chimpanzee  (anthropithecus  ni^-r).    (From  Brehm.) 

controversy  that  still  continues  as  to  whether  these  different 
forms  of  chimpanzee  and  orang  are  "  merely  local  varieties  " 
or  "  true  species  "  is  an  idle  one  ;  as  in  all  such  disputes  of 
classifiers  there  is  an  utter  absence  of  clear  ideas  as  to  what 
a  species  really  is. 

Of  the  largest  and  most  famous  of  all  the  anthropoid  apes, 
the  gorilla,  Paschen  has  lately  discovered  a  giant-form  in  the 


406 


FCETAL  MEMBRANES  AXD  CIRCULATION 


interior  of  the  Cameroons,  which  seems  to  differ  from  the 
ordinary  species  (gorilla  gina,  Fig.  242),  not  only  by  its 
unusual  size  and  strength,  but  also  by  a  special  formation  of 
the  skull.  This  giant  gorilla  (gorilla  gigas,  Figs.  243,  244) 
is  two  metres  and  seven  centimetres  [six  feet,  ten  inches]  long; 
the  span  of  its  great  arms  is  280  centimetres  [nine  feet];  its 
powerful  chest  is  twice  as  broad  as  that  of  a  strong  man. 


Fig.   241.— Female  mafuka  (antkropithe 
Cf.  R.  Hartmann's  Anthropoid  Apes,  p.  203. 


'lafuka).     (From    Brehm.) 


The  whole  structure  of  this  huge  anthropoid  ape  is  not 
merely  very  similar  to  that  of  man,  but  it  is  substantially  the 
same.  "  The  same  200  bones,  arranged  in  the  same  way, 
form  our  internal  skeleton  ;  the  same  300  muscles  effect  our 
movements  ;  the  same  hair  covers  our  skin ;  the  same  groups 
of  ganglionic  cells  compose  the  ingenious  mechanism  of  our 
brain  ;  the  same  four-chambered  heart  is  the  central  pump  of 


FCETAL  MEMBRANES  AND  CIRCULATION 


our  circulation."  The  really  existing  differences  in  the  shape 
and  size  o(  the  various  parts  are  explained  by  differences  in 

their  growth,  due  to  adaptation  to  different  habits  of  life  and 
unequal  use  o(  the  various  organs.  This  of  itself  proves 
morphologically  the  descent  of  man  from  the  ape.     We  will 


\«^SLj=^f 


Fie.  -M--.— Female  gorilla.    (From  Brehm. 


return  to  the  point  in  the  twenty-third  Chapter.  But  I 
wanted  to  point  already  to  this  important  solution  of  "the 
question  of  questions,"  because  that  agreement  in  the 
formation  of  the  embryonic  membranes  and  in  foetal  circula- 
tion which  I  have  described  affords  a  particularly  weighty 
proof  of  it.     It   is  the  more  instructive  as  even  cenogenetic 


408 


FCETAL  MEMBRANES  AND  CIRCULATION 


FlG.    243.— Male    giant-gorilla    (gorilla    gigas),    from    Yaunde, 
interior  of  the  Cameroon*.     Killed  by  H.  Paschen,  stuffed  by  Umlauff. 


FQ  TAL  MEMBRANES  AND  CIRCULATION  409 

structures  may  in  certain  circumstances  have  a  high  phylo- 
genetic  value.  In  conjunction  with  the  other  tacts,  it  affords 
a  striking  confirmation  of  our  biogenetic  law. 


Fig.  J-h.  Giant-gorilla  (gorilla  gigasj,  held  by  three  negroes,  kilK-il 
and  photographed  by  H.  Paschen  in  the  interior  of  the  Caraeroons,  al  Yaunde. 
(From  the  UmlauflF  Museum  ;ii  Hamburg,  bought  for  jo, 000  marks  by  the 
Rothschild  Museum  .u  Tring.)  Total  length  of  the  body,  from  vertex  to  middle 
:  metres  [six  feel  eight  inches]  \  the  span  of  the  outstretched  arms, 
from  one  middle-linger  to  the  other,  2.S  metres  [six  feet  nine  inches]. 


FIFTEENTH   TABLE 
SYNOPSIS  OF  THE  EMBRYONIC  PLATES 
(LAMELLAE  EMBRYONALES)  OF  THE  VERTE- 
BRATES AND  THEIR  CONNECTION  WITH  THE 
CHIEF  ORGANS  AND  TISSUES 


Germinal 

Layers. 

Blastophylls. 

Laminee  embryt 

nales. 


Germinal  Plates. 

Blastoplatts. 

Lamella  embryo- 

nales. 


Chief  Organs 

of  the 
Vertebrates. 


Tissues  of  the 
Vertebrates. 


A.  Ectoderm. 

Outer  germinal 

layer. 

Epiblast  or 

ectoblast. 

Upper  limiting 

layer. 

Skin-layer. 


i.  Horn-plate. 
Lamella  comit- 
ate. 

2.  Medullary- 
plate. 

Lamellamedul- 
laris. 

3.  Sense-plates 
(local  products 
of  the  sense- 
layer). 


Cmi^S°-      4,Cutis-plate. 


(epimera) 

dorsal 

somites. 

Primitive  se° 

merits  of  the 

dorsal  half. 

•  Stem-zone  ' 

of  the 

amniotes. 


C.  II.  Hypo- 
somites 
(hypomera) 
ventral 
somites. 
Primitive  seg- 
ments of  the 
ventral  half. 

:-  Lateral 

plates  "  ofthe 

amniotes. 


q.  Musele-plate. 
Lamella   mus- 


Skeletal- 
plate. 

Lamella  skele- 


7.  Prorenal 
canals. 
Nephrotoma. 

8.  Sexual-plate. 

Gonotoma. 

q.  Vascular- 
string's. 

fern, 

to.  Mesenterie- 
plate. 

Lamella     mes- 


B.  Entoderm. 

Inner  germinal 

layer,  hypoblast, 

or  endoblast. 

Lower  limiting 

layer. 

Gut-layer. 


/  ii.  Chorda- 
plate. 
Endoblastits 
chordalis. 


>2.  Gut  gland- 
plate. 

Lamella  ente, 


1.  Outer  skin. 
Epidermis. 

2.  Nervous 
system. 

Medullarytube, 

3.  Sense- 
organs. 
Sensilla. 


4.  Corium. 

5.  Lateral 
muscles  of 

the  trunk 
(myotomes). 

6.  Ctiorda- 
sheath     and 
its  processes 
(perichorda). 

7.  Prone- 
phridia. 

Prerenal  ca- 
nals (later  pri- 
mitive kidneys 
and  kidneys). 

■s.  Gonades' 

(ovaries     and 

spermaries). 

9.  Dorsal 
artery 

aorta 

ventral       vein 

(heart). 

10.  Mesentery 

and  muscular 
wall  of  the  gut. 


[Epithelial  Tissue  of 
'  the  outer  skin,  the 
I     mouth,  and  the 


Ganglionic  cells 
and  nerve-fibres. 


Differentiated 
sense-epithelia. 


:md 


1  Cutis,  connective 
'  tissue,  and  smooth 
I      muscles  of  the 

I     mesenchyma. 

(  Animal  muscular 
\   tissue  (striated). 

/'  Supporting  tissue 
I     of  the  skeleton, 
I      cartilage  and 

hones. 

I     Urinary  epithe- 

I  hum  of  the  prone- 
I     phridia  and  the 
later  renal  canals. 

Gonidia 

(ova  and  sper- 
matozoa). 

Tissues  of  the 

vascular   walls. 

Lymph-cells. 

Smooth  muscles 

and  mesonchvm 

ofthe  gut. 


1 .  Chorda 
(axial  rod).         I     Chorda-tissue. 

Chorda  ,1,., -sails.   \ 


12a.  Head-gut, 
Cephalogaster 

branchial  scut. 


12b.  Trunk-gut,  J 
Hepatogaster, 
Liver-eut. 


1 2a.  Respiratory 

epithelium  of  the 
gullet  and  ^ill- 
crate,  the  hypo- 
branchial  groove, 
and  the  lungs. 
-12b.  Digestive 
epithelium  of 
stomach,  liver, 
small  and  large 
intestines. 


The  Evolution  of  Man.  V.Ed. 


PLJV. 


The  Evolution  of  Man  /'/'. 


PLXV1. 


-  ■ 


EXPLANATION    OF    PLATES  XV.    AND   XVI. 

Hum. in  embryos  in  the  foetal  membranes.  The  six  figures  of  these  Plates 
are  copied  from  the  fine  steel  engravings  illustrating  The  Development  of 
Man  and  the  Chick  hi  the  Egg,  which  Professor  Erdl  (Munich)  published  in 
1845.  All  six  figures  represent  human  embryos  in  their  natural  size,  enveloped 
in  tlu'ir  membranes.  In  the  first  four  figures  from  the  second  to  the  sixth 
week  ol  development)  the  mallochorion  is  cut  away,  and  we  see  the  tiny  embryo 
enclosed  in  the  amnion.  The  small  umbilical  vesicle  (or  rudimentary  yelk- 
sac)  hangs  by  a  thin  stalk  out  ol'  the  belly  of  the  embryo,  and  lies  in  the 
n  or  seroccelom  (the  extra-embryonic  body-cavity).  (Cf.  Plate  XIV". 
and  p.  365. 1 


Plate  XV.,  Fig.  1.  A  human  embryo  with  the  total  membranes  of 
about  the  tenth  clay,  natural  size  [Erdl,  Plate  III..  Fig.  1  |. 

Plate  XV.,  Fig.  -•.  A  human  embryo  with  the  foetal  membranes  of 
about  the  fourteenth  day,  natural  size  (Erdl,  Plate  III.,  Fig.  2). 

Plate  XV.,  Fig.  3.  A  human  embryo  with  the  foetal  membranes  of 
three  weeks,  natural  size  (Erdl,  Plate  III..  Fig.   j). 

Plate  XV.,  Fig.  4.  A  human  embryo  with  the  foetal  membranes  of 
Six  weeks,  natural  size  (Erdl,  Plate  III.,  Fig.  5). 

Plate  xv..  Fig.  5.  A  human  embryo  of  twelve  weeks,  within  the 
foetal  membranes,  natural  size  (Erdl,  Plate  XI.,  Fig.  z).  The  embryo  is  com- 
pletely enclosed  in  the  amniotic  sac,  filled  with  water,  as  in  a  bath.  The 
umbilical  cord,  which  passes  from  the  navel  of  the  embryo  to  the  chorion,  is 
sheathed  with  a  continuation  of  the  amnion,  which  makes  folds  at  its  points  oi 
juncture.  Above,  the  thickly  clustered  and  branched  chorion-villi  form  the 
placenta.  The  lower  part  of  the  chorion  (cut  away  ami  lying  in  delicate  folds) 
is  smooth  and  tuftless.  Underneath  it  the  uterine  decidua,  also  cut  away  and 
spread  out,  hangs  in  coarser  folds.     Head  and  limbs  are  already  far  advanced. 


Plate  XVI.  A  human  embryo  of  five  months,  natural  size  (Erdl, 
Plate  XIV.).  The  embryo  is  enclosed  in  the  delicate,  transparent  amnion. 
which  is  evil  open  in  front,  so  that  the  face  and  limbs  stand  out.  The  back  is 
curved,  the  limbs  drawn  up.  so  that  the  embryo  takes  up  as  little  space  as 
possible  in  the  ovum.  The  eyes  are  closed.  From  the  navel  the  thick 
umbilical  cord  passes,  in  serpentine  folds,  over  the  right  shoulder  to  the  back, 
and  trout  there  to  the  spong)  placenta  (to  the  right  below).  The  thin  outer- 
most membrane,  lying  in  many  folds,  is  the  external  foetal  membrane,  the 
chorion. 


••"'Sfe 


"*WrV 


